Background: Congenital heart defects (CHD) are the most common birth defects, affecting approximately 0.8% of live births worldwide. CHD impairs the function and structure of the heart and blood vessels. Due to this damage, blood flow is impaired, which affects many major organs, including the brain, and causes various neurodevelopmental disorders.Methods: In this study, we recruited a five-generation pedigree for analysis. The proband was born with congenital heart disease and subsequently developed various neurodevelopmental disorders. To understand the causes of the disease, we performed clinical whole-exome sequencing and applied various bioinformatics tools to determine the pathogenicity of the mutation.Results: Molecular investigation revealed a novel lethal mutation (c.2321G>A) in KDM6A, causing the substitution of Glycine to Glutamic acid (Gly774Glu). The mutation was further confirmed using Sanger sequencing. Various bioinformatics tools were used to predict the lethality of the mutations. KDM6A disruption causes various diseases, among which Kabuki syndrome is the most prevalent.Conclusion: Our findings may aid in the further development of genome-based medicines, leading to a reduction in mortality rates and improved healthcare in newborns.Keywords:Congenital heart disease, Kabuki syndrome, Neurodevelopmental disorder, Whole exome sequencing, Novel mutation

The rapid global spread of SARS-CoV-2 triggered an unprecedented effort to develop effective antivirals. Among the first approved agents was remdesivir, an injectable nucleoside analogue developed by Gilead Sciences, that led to chain termination of viral RNA synthesis and showed broad antiviral activity against RNA viruses. Early clinical results were mixed: The US ACTT-1 trial reported an accelerated recovery and reduced mortality in treated patients, while the WHO Solidarity and a European trial revealed no impact of remdesivir on mortality. In contrast, a US trial in outpatients demonstrated a clear clinical benefit when treatment was administered early. Molnupiravir, an orally applicable nucleoside analogue developed by Merck, induces lethal mutations in the viral genome rather than chain termination. Molnupiravir showed invivo antiviral activity against coronaviruses in different animals. In MOVe-OUT trials, molnupiravir reduced the rate of hospitalisation in treated outpatients. In the PANORAMIC trial, molnupiravir reduced the time to recovery in outpatients but not their rate of hospitalisation. No drug effect of molnupiravir was seen in the RECOVERY trial with hospitalised COVID-19 patients. Using structural biology and medicinal chemistry approaches, Pfizer developed nirmatrelvir, an oral inhibitor of the major coronavirus protease. In high-risk but not in standard-risk COVID-19 patients, the combination nirmatrelvir/ritonavir reduced the rate of hospitalisation (EPIC HR and SR trials). Retrospective cohort studies showed treatment effects in defined patient groups. This review compares the efficacy and clinical performance of different antivirals, including emerging drugs such as obeldesivir and alternative protease inhibitors (lopinavir, simnotrelvir). It further examines their roles in prophylaxis, treatment of long covid symptoms, pharmacological considerations and antiviral resistance. Particular attention is given to factors underlying variable outcome of the trials, including viral variant evolution, population immunity increases, disease severity changes and timing of therapy initiation.

Assignment of cell fate along the dorsal/ventral (D/V) axis of the Drosophila embryo occurs during the 40 minutes immediately prior to gastrulation. The mechanism is the rapid redistribution of Decapentaplegic-Screw morphogens from lateral to dorsal regions. Redistribution requires the activity of Short gastrulation (Sog), Twisted gastrulation (Tsg), Tolloid (Tld) and Shrew. The function of Sog, Tsg and Tld and their vertebrate homologs are well known. Shrew remains an enigma, though previous studies showed that Shrew is a truncated relative of Tsg. Our phylogenetic survey revealed that Shrew is Drosophila specific and likely derived from a different Tsg family member Crossveinless. A second survey identified a correlation between insects with Shrew and those with the fastest time to gastrulation. The correlation suggests the hypothesis that Shrew functions as an accelerator of D/V axis formation as an adaption to selective pressure for rapid egg development that reduces exposure to predators. We tested the hypothesis by slowing down egg development with reduced temperature generating the prediction that an accelerator would then be unnecessary. Consistent with the prediction, at reduced temperature shrew was nonessential, with two normally lethal shrew mutants surviving to adult. Together the evolutionary and experimental data suggest that Shrew is an exemplar of a new gene that modifies a developmental process as an adaptation to an identified selective pressure. From a broader perspective the analysis of Shrew reveals that developmental processes are adaptable phenotypes, and that new gene creation is an effective response to selection.

Centrosome duplication must be tightly regulated to maintain genomic stability. In Caenorhabditis elegans, the APC/C and co-activator FZR-1 function as negative regulators of centrosome duplication by targeting specific substrates for proteolytic degradation. While C. elegans SAS-5 and ZYG-1 have been identified as substrates of APC/CFZR-1, the mechanism by which APC/CFZR-1-dependent degradation influences centrosome assembly remains unclear. Here, we identified SPD-2, the conserved homolog of human CEP192, as an APC/CFZR-1 substrate. We show that loss of APC/CFZR-1 increases both cellular and centrosomal SPD-2 levels, and that SPD-2 physically associates with FZR-1 in vivo. Functional analyses of canonical D-box motifs reveal that D-box1, D-box2, and D-box3 each contribute to SPD-2 degradation, each with different functional consequences. Mutation of D-box3 alone partially rescued zyg-1 mutant phenotypes by restoring centrosome duplication and embryonic viability through increased centrosomal SPD-2 and ZYG-1. In contrast, mutating D-box1 or D-box2 elevated cellular SPD-2 but did not rescue zyg-1, with the D-box1 mutation further reducing centrosomal SPD-2 and exacerbating duplication defects and lethality in zyg-1 mutants. Our results reveal a conserved mechanism for APC/CFZR-1-dependent degradation of SPD-2 and indicate that SPD-2 stability is regulated by multiple D-box motifs, each associated with distinct functions, linking protein stability with centrosomal localization to ensure proper centrosome assembly during C. elegans embryogenesis.

Mammals can now be cloned artificially, but it remains unknown whether they can also maintain their species through cloning. Herein, we continued serial cloning for 20 years from a single donor mouse. These re-cloned mice appeared normal and had normal lifespans, but large structural and lethal mutations accumulated in their DNA with each generation. The birth rate of serial cloning began to decline from the 27th generation, and the 58th generation was the last. When re-cloned mice from near the final generation were mated with males, their oocytes could be fertilized, but most embryos degenerated. However, a few embryos were normalized by meiosis and fertilization and developed to full term, suggesting that mammals rely on sexual rather than asexual reproduction to eliminate genetic anomalies caused by clonal reproduction.

Recessive lethal mutations are widespread across studied species, with estimates suggesting that each individual carries at least one. Numerous lethal alleles persist in wild populations at higher frequencies than expected given their extreme deleterious nature. Though these findings spurred historical debate whether classical balancing selection maintains some lethal alleles at elevated frequencies (versus mutation-selection balance acting alone), we propose the question remained unanswered, especially given that the genetic basis of most naturally occurring lethal effects is still unknown. Given current genome-wide point mutation rate estimates, mutation-selection balance alone cannot explain some of this lethal variation in nature. However, evolutionary biologists have historically studied genetic variation through a lens of single-nucleotide variants, when in fact the spectrum of mutational changes is far broader than point mutations alone, including indels, structural variants, short tandem repeats, and transposable element insertions. We uncover the genetic basis of lethality in nature and provide insight on the possible evolutionary forces allowing some to persist at higher frequencies. By locating hundreds of recessive lethal mutations in Drosophila melanogaster via complementation testing, fine-mapping, and sequencing a subset, we determine candidate lethal mutations in specific genes. We discover that many lethal disruptions are likely caused by transposable element insertions. The most common transposable elements in our data, Transib1 and Kuruka, are both estimated to have recently invaded D. melanogaster, each from a different Drosophila species (between 2013–2016 and 2017–2021, respectively). This finding demonstrates that the many lethal alleles studied in D. melanogaster in the last century had a distinct genetic basis. Hence, we propose a model that could explain lethal variation in natural populations of D. melanogaster: lethal mutation frequencies are driven by invasions of new transposable elements and as time passes after each invasion, those frequencies decline as D. melanogaster evolves suppression mechanisms, allowing for natural selection to more efficiently remove lethal insertions. Upon the invasion of a new TE, the cycle repeats. The ubiquity of lethal alleles in natural populations is a classic conundrum for evolutionary geneticists for over a century, and this study utilized modern tools and sequencing technology to provide novel insight into this age-old mystery.

Recessive lethal mutations are widespread across studied species, with estimates suggesting that each individual carries at least one. Numerous lethal alleles persist in wild populations at higher frequencies than expected given their extreme deleterious nature. Though these findings spurred historical debate whether classical balancing selection maintains some lethal alleles at elevated frequencies (versus mutation-selection balance acting alone), we propose the question remained unanswered, especially given that the genetic basis of most naturally occurring lethal effects is still unknown. Given current genome-wide point mutation rate estimates, mutation-selection balance alone cannot explain some of this lethal variation in nature. However, evolutionary biologists have historically studied genetic variation through a lens of single-nucleotide variants, when in fact the spectrum of mutational changes is far broader than point mutations alone, including indels, structural variants, short tandem repeats, and transposable element insertions. We uncover the genetic basis of lethality in nature and provide insight on the possible evolutionary forces allowing some to persist at higher frequencies. By locating hundreds of recessive lethal mutations in Drosophila melanogaster via complementation testing, fine-mapping, and sequencing a subset, we determine candidate lethal mutations in specific genes. We discover that many lethal disruptions are likely caused by transposable element insertions. The most common transposable elements in our data, Transib1 and Kuruka, are both estimated to have recently invaded D. melanogaster, each from a different Drosophila species (between 2013-2016 and 2017-2021, respectively). This finding demonstrates that the many lethal alleles studied in D. melanogaster in the last century had a distinct genetic basis. Hence, we propose a model that could explain lethal variation in natural populations of D. melanogaster: lethal mutation frequencies are driven by invasions of new transposable elements and as time passes after each invasion, those frequencies decline as D. melanogaster evolves suppression mechanisms, allowing for natural selection to more efficiently remove lethal insertions. Upon the invasion of a new TE, the cycle repeats. The ubiquity of lethal alleles in natural populations is a classic conundrum for evolutionary geneticists for over a century, and this study utilized modern tools and sequencing technology to provide novel insight into this age-old mystery.

F-box proteins of SCF E3 ligases have been documented to control the abundance of numerous critical regulatory proteins. In Arabidopsis, one of them, F-BOX-LIKE17 (FBL17), stands out for playing a key role in DNA replication, DNA damage, and, more recently, for the control of cell size. FBL17 null mutants exhibit severe cellular defects leading to lethality. However, the molecular mechanisms by which FBL17 operate remain poorly understood. Here, we show that FBL17 interacts with different components of the RETINOBLASTOMA-RELATED1/E2F module and is involved in the protein turnover of E2Fa and E2Fb. However, mutations in E2Fa or E2Fb do not alleviate the severe fbl17 phenotype but worsen it. By contrast, it is the accumulation of the transcriptional repressor E2Fc that causes fbl17 mutant lethality. Our results highlight a key role for FBL17 in modulating the transcriptional control of E2F target genes ensuring precise control of cell cycle progression and avoiding uncontrolled DNA damage response.

The CRISPR-Cas9 technology is a powerful tool for targeted modifications of DNA sequences with unprecedented precision. In enabling researchers with the ability to fix certain lethal and non-lethal mutations that cause diseases like Duchenne muscular dystrophy, sickle cell anaemia, and beta-thalassemia, this powerful technology has shown promise in treatment of these genetic conditions. Several ethical questions have been raised about using CRISPR-Cas9, the most concerning being the editing of human embryos, particularly those pertaining to germline modifications that can alter genes in subsequent generations. Furthermore, the creation of so-called "designer babies" raises ethical questions about consent, autonomy, and potential damage from unintended off-target effects. The possibility that CRISPR-Cas9 may be used for purposes other than medicine, such as altering physical characteristics and intellect, is another concern. The main aim of this review is to curate updated literature on CRISPR-Cas9 technology, its potential for use in genetic therapies, and the ethical and legal issues associated with it. The review also discusses the need for robust ethical framing and regulatory oversight, assuring that gene-editing technologies will be responsible and equitable, with consideration of human dignity and diversity.

Invasive pests threaten food security and devastate ecosystems. A universal problem in their management is that small populations can easily evade detection. This makes identifying new incursions challenging and complicates efforts to eradicate or contain established populations. If newly founded populations exhibited a strong Allee effect, small populations would tend towards extinction and most new incursions would go extinct without the need for detection or intervention. Of course, invasive species rarely exhibit strong Allee effects, but new genetic technologies make it conceivable to impose one. Here we consider how introduction of genetic load can cause a genetic Allee effect that reduces the establishment probability of small founder populations. Using numerical and individual-based modelling, we examine the fate of populations sampled from a larger invasive source population carrying deleterious recessive alleles. Our analysis reveals that the genetic load unmasked by founding can dramatically reduce the establishment probability of small populations across a wide range of parameter space. A sterile mutation effect is more effective than a lethal mutation effect, but X-linkage offers minimal benefit over autosomal inheritance. Although extinction of newly founded populations is a common outcome, it may be challenging to achieve in species with very high reproductive outputs. Distributing deleterious recessive alleles across a large number of loci at low frequencies was more effective than distributing them across fewer loci at higher frequencies. Our findings suggest that driving deleterious recessives into a source population may render it less prone to establish in new areas.

Understanding how mutations affect protein function remains critical yet challenging, particularly for variants in clinical databases lacking experimental characterisation and for intrinsically disordered regions. Current computational approaches often operate as black boxes, providing predictions without sufficient transparency or quality assessment of the underlying data. Here we present ProteoCast, a user-friendly web server that predicts variant effects through evolutionary constraint analysis and structural context integration. ProteoCast provides a three-tier variant classification (impactful, mild, neutral) to help prioritise mutations for clinical interpretation and experimental validation. It incorporates multiple sequence alignment quality controls to ensure prediction reliability and flag positions with insufficient evolutionary information. Beyond single-variant classification, ProteoCast employs a novel segmentation approach based on mutational sensitivity to identify functional linear peptides in disordered regions. Interactive visualisations guide users through results interpretation, from variant-level predictions to protein-wide functional landscapes. Evaluation on 63,000 ClinVar variants demonstrates 77% sensitivity and 87% specificity for pathogenicity prediction, with performance maintained across species (85% accuracy on Drosophila lethal mutations). ProteoCast successfully identifies twice as many functional motifs in intrinsically disordered regions compared to conservation-based phylogenetic methods. Predictions can be tuned to specific conformations, such as bound forms in protein complexes, for improved accuracy and interpretability. With its transparent, unsupervised methodology and computational efficiency (minutes per protein), ProteoCast democratises access to variant effect prediction and functional site discovery for the broader research community. The web server is freely available at: https://proteocast.ijm.fr/.

Inbreeding, or close-relative mating, is a traditional and highly effective tool in the selection of farm animals, primarily aimed at fixing desirable economic traits. However, uncontrolled and intensive inbreeding carries significant genetic risks that fundamentally alter the genotypic structure of a population. The primary genetic consequence is a sharp increase in homozygosity, which inevitably leads to the expression of harmful, sublethal, or lethal recessive mutations. This phenomenon, known as inbreeding depression, often severely impairs both biological and economic performance. This study analyzes the theoretical genetic aspects of inbreeding and the phenomenon of inbreeding depression in the context of commercial animal husbandry. Furthermore, we present a series of clinical cases of severe congenital malformations-namely, cyclopia, holoprosencephaly, and otocephaly - documented in various farm animal species, including horses, sheep/goats, and rabbits. These severe anomalies, often fatal and associated with mutations in key developmental regulatory genes, are used as direct indicators of genetic risk. The described cases provide concrete evidence of the increased vulnerability of populations with a narrow gene pool to the manifestation of rare recessive genetic defects. The research materials include a review of classical genetic models, statistical data on the prevalence of anomalies, and documented clinical cases, including our own results from full-sibling crossings in rabbits. The results of this integrated analysis underscore the necessity of meticulous genetic monitoring and the implementation of molecular screening in all breeding populations. We recommend maintaining the inbreeding coefficient below 5 – 6 % over five generations, consistently rotating unrelated sires, and systematically excluding known carriers of lethal recessive mutations to mitigate the negative consequences of genetic load.

Centrosome duplication must be tightly regulated to maintain genomic stability. In Caenorhabditis elegans, the APC/C and co-activator FZR-1 function as negative regulators of centrosome duplication by targeting specific substrates for proteolytic degradation. While C. elegans SAS-5 and ZYG-1 have been identified as substrates of APC/CFZR−1, the mechanism by which APC/CFZR−1-dependent degradation influences centrosome assembly remains unclear. Here, we identified SPD-2, the conserved homolog of human CEP192, as a substrate of APC/CFZR-1. We show that loss of APC/CFZR−1 increases both cellular and centrosomal SPD-2 levels, and that SPD-2 physically associates with FZR-1 in vivo. Functional analyses of canonical D-box motifs reveal that D-box1, D-box2, and D-box3 each contribute to SPD-2 degradation, each with different functional consequences. Mutation of D-box3 alone partially rescued zyg-1 mutant phenotypes by restoring centrosome duplication and embryonic viability through increased centrosomal SPD-2 and ZYG-1. In contrast, mutating D-box1 or D-box2 elevated cellular SPD-2 but did not rescue zyg-1, with the D-box1 mutation further reducing centrosomal SPD-2 and exacerbating duplication defects and lethality in zyg-1 mutants. Our results reveal a conserved mechanism for APC/CFZR−1-dependent degradation of SPD-2 and show that its degron motifs have dual functions in degradation and centrosomal localization, ensuring robust control of centrosome assembly during C. elegans embryogenesis.

Multidrug-resistant (MDR) Streptococcus suis (S. suis) is a zoonotic pathogen capable of infecting pigs across all age groups, leading to conditions such as meningitis, arthritis, and endocarditis. In humans, infections can result in septic arthritis, meningitis, necrotizing fasciitis, and septicemia, which may be fatal. The absence of a complete genome sequence hinders comprehensive bioinformatic studies of MDR S. suis derived from pigs. In this study, we present the whole-genome sequence of MDR S. suis serotype 2 ST01 isolated from joint fluid samples obtained from pigs. Whole-genome analysis revealed that the ST01 chromosome carries 19 antibiotic resistance genes that confer resistance to major classes of antibiotic including aminoglycosides, tetracyclines, fluoroquinolones, lincosamides, polypeptide, and nitrofurans. Additionally, it contains 15 virulence factors associated with immune modulation, bacterial adherence, and stress survival. Whole-genome analysis identified 84 horizontal gene transfer elements in ST01 (comprising 28 genomic islands, 52 transposons, and 4 prophages), alongside mutations resulting in reduced virulence (302 instances) and loss of pathogenicity (34 instances). Furthermore, 18 antibiotic targets along with 21 lethal mutations were identified as potential targets for preventing, controlling, and treating infection caused by MDR S. suis serotype 2 ST01. In vivo infection experiments demonstrated that intraperitoneal inoculation with ST01 resulted in mortality among Kunming mice, with a median lethal dose (LD50) of 5.62 × 109 CFU/mL. Histopathological analysis revealed varying degrees of lesions in the infected organs of the mice. This study thus provides valuable insights into strategies aimed at combating S. suis infections and their transmission within swine populations.

The Sterile Insect Technique (SIT) is an environmentally friendly, sustainable pest control approach, which uses large-scale releases of sterile insects to suppress or eradicate target populations through infertile matings. The efficiency of SIT is enhanced by male-only releases requiring genetic sexing strains (GSSs) that are classically based on selectable recessive visible markers or temperature-sensitive lethal (tsl) mutations and a rescue by a wild-type allele translocated to the male-determining chromosome. The transfer of identified or designed temperature-sensitive alleles might allow the generation of neoclassical GSSs in additional SIT target species. By using precise genome-editing tools, such as CRISPR/Cas, the creation of specific mutations in target genes and the integration of a wild-type copy is feasible without the introduction of foreign DNA. This might ease regulation of neoclassical GSSs, since they are not considered transgenic. However, integration and expression of genes at male-determining loci or chromosomes is not reliably established. Therefore, additional strategies to link temperature-sensitive phenotypes to female development are required, which could be achieved by targeting genes involved in dosage compensation or sex determination. To create temperature-sensitive alleles, rational protein design using advanced modeling and prediction tools to evaluate and tailor the effect of mutations on protein stability and temperature sensitivity can be used. In addition, emerging synthetic biology strategies such as temperature-inducible N-degrons or temperature-sensitive inteins provide powerful tools to generate temperature sensitivity. Such approaches should enable conditional control over proteins causing female lethality or sex conversion and therefore promise straightforward generic approaches to generate GSSs for male-only production in SIT target species.

Leaf color variations in tea plants are economically significant, with inheritable albino mutants being key models for investigating the mechanisms around chloroplast biogenesis and development. However, lethal mutants were rarely observed in tea plants. In this study, we simultaneously discovered the etiolated mutants (Y) and the novel albino lethal variant (A) during seed germination trials of the tea plant cultivar "Fuding Dabai." This unprecedented albino lethal mutant (A) is distinguished from the previously reported albino mutants of tea plants by its white cotyledons and an inability to sustain growth following the exhaustion of cotyledonary nutrients. Our findings indicated a significant reduction in chlorophyll a, chlorophyll b, α-carotene, and β-carotene in the albino lethal mutants compared to the etiolated mutants and normal green tea plants. Interestingly, the total amino acid content was notably elevated in the lethal albino mutant. Transmission electron microscopy revealed impaired chloroplasts in both the etiolated mutants and the albino lethal mutants. RNA transcriptome sequencing identified 3428 differentially expressed genes (DEGs) in the comparison of the lethal albino mutant vs. the wild type, with 1453 upregulated and 1975 downregulated DEGs, highlighting the genetic underpinnings of chloroplast development, pigment biosynthesis and amino acid metabolism, which was further confirmed by real-time quantitative PCR analysis. This study provides a detailed characterization of a lethal albino mutant caused by chloroplast development deficiency in tea plants, offering insights into the genetic mechanisms of albinism and setting the stage for future research on leaf color variation and chloroplast morphogenesis in plants.

SUMMARYLEUNIG (LUG) and LEUNIG_HOMOLOG (LUH) are Groucho/Tup1‐type transcriptional co‐regulators in Arabidopsis thaliana that act redundantly across multiple developmental and environmental response pathways. Their specific contributions to flower development, however, have remained unclear due to embryonic lethality of double mutants. Here, we show that LUH associates with distinct sets of transcription factors and chromatin‐associated proteins in a developmentally dynamic manner. Chromatin occupancy and protein interactions shift from a meristem‐focused network during early floral patterning to an organogenesis‐oriented state as primordia initiate and expand. Reduced promoter‐proximal LUH binding coincides with this proliferative phase, suggesting a transient reconfiguration of LUH activity. By later stages, LUH activity is amplified alongside organ differentiation programs. Together with LUG, LUH modulates gene expression programs that are essential for establishing floral organ patterning. These findings reveal how dynamic co‐regulator assemblies contribute to the temporal coordination of growth and spatial pattern formation in Arabidopsis flowers.

Beneficial mutations drive the within-host adaptation of viral populations and can prolong the duration of host infection. Yet, most mutations are not adaptive and the increase of the mean fitness of viral populations is hampered by deleterious and lethal mutations. Because of this ambivalent role of mutations, it is unclear if a higher mutation rate boosts or slows down viral adaptation. Here, we study the interplay between selection, mutation, genetic drift and within-host dynamics of viral populations. We obtain good approximations for the transient evolutionary epidemiology of viral adaptation under the assumption that the mutation rate is high and the effects of nonlethal mutations remain small. We use measures of fitness effects of mutations for a range of viruses to predict the critical mutation rate required to drive viral extinction. This analysis questions the feasibility of lethal mutagenesis because the fold increase of viral mutation rates induced by available mutagenic drugs is not high enough to reach the critical mutation rate predicted by our model.

Lethal yellow mutation in the Raly-Agouti locus reduces the energy expenditure in male mice.

Sterol biosynthesis in eukaryotes is commonly regulated by Sterol Regulatory Element Binding Proteins (SREBPs), membrane-bound transcription factors activated by proteolytic cleavage in response to sterol levels. While extensively studied in mammals, SREBP function and regulation in fungi remain less understood. Here, we identify and characterize two SREBP homologs, Hms1-1 and Hms1-2, in the methylotrophic yeast Komagataella phaffii. Transcriptional and physiological analyses reveal that Hms1-1 and Hms1-2, together with Upc2, form a central regulatory hub controlling sterol-responsive gene expression. Despite largely unchanged total sterol levels, the double deletion mutant (hms1-1Δ hms1-2Δ) exhibits altered expression of sterol biosynthesis genes and increased sensitivity to terbinafine, indicating a sterol-responsive role for SREBPs. Remarkably, K. phaffii retains both Upc2 and SREBP pathways, an unusual dual regulatory system in fungi that highlights evolutionary plasticity. Functional assays demonstrate that overexpression of either HMS1-1 or HMS1-2 rescues the lethality of upc2Δ mutants, revealing redundancy and flexibility in sterol regulation. We show that proteolytic activation of these homologs depends on Scp, a functional analog of mammalian Scap, and the Dsc E3 ligase complex. Comparative analyses identify distinct roles: Hms1-2 acts as the primary, Scp-dependent activator, whereas Hms1-1 is constitutively processed, less Scp-dependent, and likely regulated by post-translational sumoylation. These findings establish K. phaffii as a genetically tractable model for studying SREBP signaling and provide new insights into the evolution and complexity of fungal sterol homeostasis.