Broken Ends Research Articles

Pre-exposure to ionizing radiation stimulates DNA double strand break end resection, promoting the use of homologous recombination repair.

The choice of DNA double strand break (DSB) repair pathway is determined at the stage of DSB end resection. Resection was proposed to control the balance between the two major DSB repair pathways, homologous recombination (HR) and non-homologous end joining (NHEJ). Here, we examined the regulation of DSB repair pathway choice at two-ended DSBs following ionizing radiation (IR) in G2 phase of the cell cycle. We found that cells pre-exposed to low-dose IR preferred to undergo HR following challenge IR in G2, whereas NHEJ repair kinetics in G1 were not affected by pre-IR treatment. Consistent with the increase in HR usage, the challenge IR induced Replication protein A (RPA) foci formation and RPA phosphorylation, a marker of resection, were enhanced by pre-IR. However, neither major DNA damage signals nor the status of core NHEJ proteins, which influence the choice of repair pathway, was significantly altered in pre-IR treated cells. Moreover, the increase in usage of HR due to pre-IR exposure was prevented by treatment with ATM inhibitor during the incubation period between pre-IR and challenge IR. Taken together, the results of our study suggest that the ATM-dependent damage response after pre-IR changes the cellular environment, possibly by regulating gene expression or post-transcriptional modifications in a manner that promotes resection.

The night before the first day of school.

DOI: 10.1200/JCO.2014.60.5790 So many years out of school and that feeling returns again. Part nausea, part excitement, part dread, part wonder that another summer is over and another school year begins. It is the night before the first day of school. As a student, the end of summer break somehow became easier to handle with each passing year. Maybe it was maturity or experience or confidence that the school year would be OK. Or maybe I just learned to accept the inevitable. Now, as a parent with that anxiety and opportunity once removed, each school start brings unique perspectives from each of my children. And like so many things, through my children, it is easy to feel school-aged again. I am hoping that the teachers will be nice, wondering if any of my children’s friends will be in home room, and curious about who will have lunch at the same time. And I am more worried than ever that the teachers will give too much homework. As a pediatrician, my patients provide a unique perspective on school. Working in the emergency room many years ago, I was helping a very sick child— getting oxygen for him, putting in an intravenous tube, and handing off my other patients so I could focus on this one sick child. Amid all of the rushing to help him, he turned to me and asked me to get his school bag. He said he needed to study and do his homework. I told him that it would be OK to take the night off, but this 11-year-old boy from the inner city removed his oxygen mask to tell me, “Non scooli sed vitae diskemuse” (Fig 1). I figured he was babbling from too little oxygen going to his brain. But when I looked confused, he said, “You’re a doctor. Don’t you know Latin? It means ‘Not for school but for life we learn.’” If only I had learned that when I was his age. . . It is once again the night before the first day of the new school year. My own children and other kids across the country are clinging to the last hours of summer break. But as a pediatric oncologist, I know one 15-year-old girl is hoping for morning to arrive so she can see her friends, and be a kid, and maybe learn something, not for school, but for life. Christy has a recurrent malignant brain tumor that no one knows how to cure. For patients with the worst types of tumors, we try to set realistic goals. What are some of the things you want to do while you can? Amazingly, most of our patients, even children and teenagers, can answer this question. After her high-grade glioma recurred the first time, Christy danced at the prom party we throw at the hospital for all of our patients. And I mean, she danced. After her tumor recurred a second time, we thought it would be a small blessing if she could be strong enough to make it to see her favorite boy-band in concert. And she did, first class, limo and all, thanks to some great philanthropy. And then her tumor progressed and before she was too weak to get around, we asked what else she was hoping to do. The typical answers are “another trip” or “shopping spree” or “I want to be there for my cousin’s wedding.” But Christy’s great wish was to make it to the first day of school, the first day of 10th grade. So tonight, my boys are tucked in and hopefully looking forward with more excitement than dread to another year of school. And they and I know they will be OK. This year the transition will be easier than last year, and next year will be easier still. But I have never been more anxious before the first day of school than I am right now. I am Fig 1. Photo of Latin quote from 11-year old patient. JOURNAL OF CLINICAL ONCOLOGY A R T O F O N C O L O G Y VOLUME 33 NUMBER 13 MAY 1 2015

Functions of Fun30 chromatin remodeler in regulating cellular resistance to genotoxic stress.

The Saccharomyces cerevisiae Fun30 chromatin remodeler has recently been shown to facilitate long-range resection of DNA double strand break (DSB) ends, which proceeds homologous recombination (HR). This is believed to underlie the role of Fun30 in promoting cellular resistance to DSB inducing agent camptothecin. We show here that Fun30 also contributes to cellular resistance to genotoxins methyl methanesulfonate (MMS) and hydroxyurea (HU) that can stall the progression of DNA replication. We present evidence implicating DNA end resection in Fun30-dependent MMS-resistance. On the other hand, we show that Fun30 deletion suppresses the MMS- and HU-sensitivity of cells lacking the Rad5/Mms2/Ubc13-dependent error-free DNA damage tolerance mechanism. This suppression is not the result of a reduction in DNA end resection, and is dependent on the key HR component Rad51. We further show that Fun30 negatively regulates the recovery of rad5Δ mutant from MMS induced G2/M arrest. Therefore, Fun30 has two functions in DNA damage repair: one is the promotion of cellular resistance to genotoxic stress by aiding in DNA end resection, and the other is the negative regulation of a Rad51-dependent, DNA end resection-independent mechanism for countering replicative stress. The latter becomes manifest when Rad5 dependent DNA damage tolerance is impaired. In addition, we find that the putative ubiquitin-binding CUE domain of Fun30 serves to restrict the ability of Fun30 to hinder MMS- and HU-tolerance in the absence of Rad5.

The analysis of 54 cases of anastomasis for traumatic lacrimal canaliculus lacerations

Objective To investigate the effective methods of anastomosis for traumatic lacrimal canaliculus lacerations. Methods The appropriate surgical methods of broken ends finding and anastomosis were performed in 54 eyes of 54 cases with traumatic lacrimal canaliculus lacerations according to their specific injuries. Results Of 54 eyes after treatment 4 cases of 48 eyes with inferior lacrimal canaliculus laceration were obstructed in early time postoperatively, 4 cases obstructed 6 months later. One case of 3 patients with superior lacrimal canaliculus laceration appeared epiphora. 3 cases with both superior and inferior laceration of lacrimal canaliculus flushed smoothly when the pipes were put out. Conclusion Appropriate surgical method according to injury condition is effective for traumatic lacrimal canaliculus laceration. Key words: Lacrimal canalicular, lacerations, traumatic; Anastomosis

Dynamic failure behaviour of blended spun yarns during winding

In the present work, the initiative is being taken to study the spun yarn failure behaviour under real dynamic conditions i.e. winding. An attempt is made to study the dynamic failure behaviour of blended ring yarn in relation to yarn structure. Multi-coloured tracer fibre technique is employed to study the broken ends of yarn during the winding process in terms of proportion of fibre break/slip, configuration of yarn broken end and yarn failure length. The ring spun yarn failure during winding is basically dominated by fibre breakage. The length of broken and slipped fibres is found to be different in trailing and leading portion of the broken ends and also depends on viscose content in the blends. The failure zone length and sum of percentage of tapered and slipped broken ends are found to follow the exactly similar trend with fibre failure coefficient with the change in blend ratio. The mathematical model established to explain the spun yarn failure during winding process displays lower error%.

The Clacton Spear: The Last One Hundred Years

In 1911, an eminent amateur prehistorian pulled the broken end of a pointed wooden shaft from Palaeolithic-age sediments at a seaside town in Essex. This artefact, still the earliest worked wood to be discovered in the world, became known as the Clacton spear. Over the past one hundred years it has variously been interpreted as a projectile weapon, a stave, a digging stick, a snow probe, a lance, a game stake and a prod to ward off rival scavengers. These perspectives have followed academic fashions and as the popular views of early hominins have altered. Since discovery, the Clacton spear has also been replicated twice, has undergone physical transformations due to preservation treatments, and has featured in two public exhibitions. Within this article the changing context of the spear, its parallels, and all previous conservation treatments and their impacts are assessed.

Dimensional and defectivity nanometrology of directed self‐assembly patterns

AbstractDefectivity and dimensional metrology are two main challenges in lithography due to the increasing miniaturisation of circuits. Particularly, bottom‐up alternative lithographic masks from directed self‐assembly systems have been extending the limits of critical dimensions in a cost‐effective manner although great challenges in controlling defectivity remain open. To gain insights about the percentage of alignment, defectivity and order quantification, block copolymer fingerprints were investigated via an image analysis methodology. Here we present the analysis of hexagonal phase of polystyrene‐b‐polydimethylsiloxane (PS‐b‐PDMS) forming linear patterns in topological substrates. From our methodology, we have performed dimensional metrology estimating pitch size and error, and the linewidth of the lines was estimated. In parallel, the methodology allowed us identification and quantification of typical defects observable in self‐assembly, such as turning points, disclination or branching points, break or lone points and end points. The methodology presented here represents a step forward in dimensional metrology and defect analysis of self‐ and directed assembly systems. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Escape of Sgs1 from Rad9 inhibition reduces the requirement for Sae2 and functional MRX in DNA end resection.

Homologous recombination requires nucleolytic degradation (resection) of DNA double-strand break (DSB) ends. In Saccharomyces cerevisiae, the MRX complex and Sae2 are involved in the onset of DSB resection, whereas extensive resection requires Exo1 and the concerted action of Dna2 and Sgs1. Here, we show that the checkpoint protein Rad9 limits the action of Sgs1/Dna2 in DSB resection by inhibiting Sgs1 binding/persistence at the DSB ends. When inhibition by Rad9 is abolished by the Sgs1-ss mutant variant or by deletion of RAD9, the requirement for Sae2 and functional MRX in DSB resection is reduced. These results provide new insights into how early and long-range resection is coordinated.

Interdependence of the Rad50 Hook and Globular Domain Functions

Rad50 contains a conserved Zn(2+) coordination domain (the Rad50 hook) that functions as a homodimerization interface. Hook ablation phenocopies Rad50 deficiency in all respects. Here, we focused on rad50 mutations flanking the Zn(2+)-coordinating hook cysteines. These mutants impaired hook-mediated dimerization, but recombination between sister chromatids was largely unaffected. This may reflect that cohesin-mediated sister chromatid interactions are sufficient for double-strand break repair. However, Mre11 complex functions specified by the globular domain, including Tel1 (ATM) activation, nonhomologous end joining, and DNA double-strand break end resection were affected, suggesting that dimerization exerts a broad influence on Mre11 complex function. These phenotypes were suppressed by mutations within the coiled-coil and globular ATPase domains, suggesting a model in which conformational changes in the hook and globular domains are transmitted via the extended coils of Rad50. We propose that transmission of spatial information in this manner underlies the regulation of Mre11 complex functions.

Quantitative Evaluation of Growth Plates around the Knees of Adolescent Soccer Players by Diffusion-Weighted Magnetic Resonance Imaging.

Purpose. To quantitatively evaluate growth plates around the knees in adolescent soccer players utilizing the diffusion-weighted MR imaging (DWI). Methods. The knees and adjacent growth plates of eleven 14-year-old male soccer players were evaluated by MRI before (end of season's summer break) and after two months of intense soccer training. MRI evaluation was conducted in coronal plane by PD-FSE and DWI. All images were screened for any major pathological changes. Later, central growth plate surface area (CGPSA) was measured and the apparent diffusion coefficient (ADC) values were calculated in two most central coronal slices divided into four regions: distal femur medial (DFM), distal femur lateral (DFL), proximal tibia medial (PTM), and proximal tibia lateral (PTL). Results. No gross pathology was diagnosed on MRI. CGPSA was not significantly reduced: DFM 278 versus 272, DFL 265 versus 261, PTM 193 versus 192, and PTL 214 versus 210. ADC decrease was statistically significant only for PTM: DFM 1.27 versus 1.22, DFL 1.37 versus 1.34, PTM 1.13 versus 1.03 (p = 0.003), and PTL 1.28 versus 1.22. Conclusions. DWI measurements indicate increased cellularity in growth plates around knees in footballers most prominent in PTM after intense training. No detectable differences on a standard PD-FSE sequence were observed.

Nuclear localization of mouse Ku70 in interphase cells and focus formation of mouse Ku70 at DNA damage sites immediately after irradiation.

To elucidate the mechanisms of DNA repair pathway is critical for developing next-generation radiotherapies and chemotherapeutic drugs for cancer. Ionizing radiation and many chemotherapeutic drugs kill tumor cells mainly by inducing DNA double-strand breaks (DSBs). The classical nonhomologous DNA-end joining (NHEJ) (C-NHEJ) pathway repairs a predominant fraction of DSBs in mammalian cells. The C-NHEJ pathway appears to start with the binding of Ku (heterodimer of Ku70 and Ku80) to DNA break ends. Therefore, recruitment of Ku to DSB sites might play a critical role in regulating NHEJ activity. Indeed, human Ku70 and Ku80 localize in the nuclei and accumulate at microirradiated DSB sites. However, the localization and regulation mechanisms of Ku70 and Ku80 homologues in animal models, such as mice and other species, have not been elucidated in detail, particularly in cells immediately after microirradiation. Here, we show that EYFP-tagged mouse Ku70 localizes in the interphase nuclei of mouse fibroblasts and epithelial cells. Furthermore, our findings indicate that EYFP-mouse Ku70 accumulates with its heterodimeric partner Ku80 immediately at laser-microirradiated DSB sites. We also confirmed that the structure of Ku70 nuclear localization signal (NLS) is highly conserved among various rodent species, such as the mouse, rat, degu and ground squirrel, supporting the idea that NLS is important for the regulation of rodent Ku70 function. Collectively, these results suggest that the mechanisms of regulating the localization and accumulation of Ku70 at DSBs might be well conserved between the mouse and human species.

Spectroscopic Studies of Nanocomposites Based on PEO/PVDF Blend Loaded by SWCNTs

Novel nanocomposites of PEO/PVDF loaded by different contents of SWCNTs were prepared. From UV-Vis spectra, the estimated values of absorption edge, absorption coefficient, energy of tail and extinction coefficient were calculated. The exponential decay tail at low energy is due to the presence of localized states in the energy band gap. The shift of Raman spectra indicates the possibility of SWCNTs to be attached with OH group in the side chain of polymer molecules. The X-ray measurements indicate that the addition of SWCNTs is a rich amorphous region in polymeric matrices and complete complexation between PEO/PVDF blend and SWCNTs. The SEM images show smooth surface of PEO/PVDF and they become rough with single rod with a good distribution within surface region in polymeric matrices after loading SWCNTs. TEM images showed that SWCNTs were dispersed well in polymeric matrices. The broken ends of nanotubes were showing that the SWCNTs were broken apart rather than from at the fracture surface. The films show some defects like rags on the tube walls. There are some black regions in TEM images attributed to amorphous nature carbon. The prepared nanocomposites are very useful in technical application.

Clinical analysis of surgical technique in 41 cases of inferior lacrimal canaliculus laceration

Objective To analyze the surgical techniques of lacrimal canaliculus laceration. Methods 41 cases of inferior lacrimal canaliculus laceration received the anastomosis. The data of patients were analysed retrospectively. There are three key steps during the operation： searching the nasal broken end, supporting with the silicone stent tube and anastomosing by the way of ＂one-stich anastomosis through the skin＂. Results All cases were followed up for 3 months to 1 year after the operation. The 38 cases of inferior lacrimal canaliculus laceration were cured, accounting for 92.7%. The surgery was effective in 3 cases （7.3%） , and was invalid in no cases. Conclusion Finding the broken end, appropriate supporting, and anastomosis accurate are keys to repair lacrimal canaliculus laceration. Key words: Laceration, inferior lacrimal canaliculus; Stent, silicon; Anastomosis, one-stich, through the skin

Efficient processing of abasic sites by bacterial nonhomologous end-joining Ku proteins.

Intracellular reactive oxygen species as well as the exposure to harsh environmental conditions can cause, in the single chromosome of Bacillus subtilis spores, the formation of apurinic/apyrimidinic (AP) sites and strand breaks whose repair during outgrowth is crucial to guarantee cell viability. Whereas double-stranded breaks are mended by the nonhomologous end joining (NHEJ) system composed of an ATP-dependent DNA Ligase D (LigD) and the DNA-end-binding protein Ku, repair of AP sites would rely on an AP endonuclease or an AP-lyase, a polymerase and a ligase. Here we show that B. subtilis Ku (BsuKu), along with its pivotal role in allowing joining of two broken ends by B. subtilis LigD (BsuLigD), is endowed with an AP/deoxyribose 5′-phosphate (5′-dRP)-lyase activity that can act on ssDNA, nicked molecules and DNA molecules without ends, suggesting a potential role in BER during spore outgrowth. Coordination with BsuLigD makes possible the efficient joining of DNA ends with near terminal abasic sites. The role of this new enzymatic activity of Ku and its potential importance in the NHEJ pathway is discussed. The presence of an AP-lyase activity also in the homolog protein from the distantly related bacterium Pseudomonas aeruginosa allows us to expand our results to other bacterial Ku proteins.

Exo1 phosphorylation status controls the hydroxyurea sensitivity of cells lacking the Pol32 subunit of DNA polymerases delta and zeta.

Exo1 belongs to the Rad2 family of structure-specific nucleases and possesses 5'-3' exonuclease activity on double-stranded DNA substrates. Exo1 interacts physically with the DNA mismatch repair (MMR) proteins Msh2 and Mlh1 and is involved in the excision of the mispaired nucleotide. Independent of its role in MMR, Exo1 contributes to long-range resection of DNA double-strand break (DSB) ends to facilitate their repair by homologous recombination (HR), and was recently identified as a component of error-free DNA damage tolerance pathways. Here, we show that Exo1 activity increases the hydroxyurea sensitivity of cells lacking Pol32, a subunit of DNA polymerases δ and ζ. Both, phospho-mimicking and dephospho-mimicking exo1 mutants act as hypermorphs, as evidenced by an increase in HU sensitivity of pol32Δ cells, suggesting that they are trapped in an active form and that phosphorylation of Exo1 at residues S372, S567, S587, S692 is necessary, but insufficient, for the accurate regulation of Exo1 activity at stalled replication forks. In contrast, neither phosphorylation status is important for Exo1's role in MMR or in the suppression of genome instability in cells lacking Sgs1 helicase. This ability of an EXO1 deletion to suppress the HU hypersensitivity of pol32Δ cells is in contrast to the negative genetic interaction between deletions of EXO1 and POL32 in MMS-treated cells as well as the role of EXO1 in DNA-damage treated rad53 and mec1 mutants.

Telomere-dependent genomic integrity: evolution of the fusion-bridge-breakage cycle concept.

Most cancer genomes show abnormalities in chromosome structure and number, two types of aberrations that could share a common mechanistic origin through proliferation-dependent loss of telomere function. Impairment of checkpoints that limit cell proliferation when telomeres are critically short might allow unrestrained cell division. The resulting uncapped chromosomes can fuse to each other, forming unstable configurations that can bridge during mitosis. Chromatin bridges can break to generate new broken ends that will then fuse with other broken ends. Successive events of break and fusion will continuously generate unbalanced chromosomal rearrangements, leading to gene-copy gains and losses. However, chromosome bridges do not always break. Evidence has recently been obtained to suggest that telomere-dependent chromosome bridges remaining unbroken can hinder cytokinesis and yield tetraploid cells. This might constitute an unstable intermediate in tumorigenesis, as progressive losses of individual chromosomes due to geometrical defects during cell division result in subtetraploid karyotypes. Additionally, the presence of short dysfunctional telomeres in cells can also cause these cells to become sensitive to mutagens, and particularly to radiation exposure. Human individuals exhibit differences in their sensitivity to radiation, which can be relevant for choice of therapy. Telomere function may well be involved in cellular and organism responses to ionizing radiation. Since eroded telomeres are sensed and act as double-strand breaks, they can interact with radiation-induced breaks, sharply increasing the possibility of misjoining. Altogether, this scenario provides certain clues to understanding the important role of telomeres in maintaining genomic integrity.

Mechanism of suppression of chromosomal instability by DNA polymerase POLQ.

Although a defect in the DNA polymerase POLQ leads to ionizing radiation sensitivity in mammalian cells, the relevant enzymatic pathway has not been identified. Here we define the specific mechanism by which POLQ restricts harmful DNA instability. Our experiments show that Polq-null murine cells are selectively hypersensitive to DNA strand breaking agents, and that damage resistance requires the DNA polymerase activity of POLQ. Using a DNA break end joining assay in cells, we monitored repair of DNA ends with long 3′ single-stranded overhangs. End joining events retaining much of the overhang were dependent on POLQ, and independent of Ku70. To analyze the repair function in more detail, we examined immunoglobulin class switch joining between DNA segments in antibody genes. POLQ participates in end joining of a DNA break during immunoglobulin class-switching, producing insertions of base pairs at the joins with homology to IgH switch-region sequences. Biochemical experiments with purified human POLQ protein revealed the mechanism generating the insertions during DNA end joining, relying on the unique ability of POLQ to extend DNA from minimally paired primers. DNA breaks at the IgH locus can sometimes join with breaks in Myc, creating a chromosome translocation. We found a marked increase in Myc/IgH translocations in Polq-defective mice, showing that POLQ suppresses genomic instability and genome rearrangements originating at DNA double-strand breaks. This work clearly defines a role and mechanism for mammalian POLQ in an alternative end joining pathway that suppresses the formation of chromosomal translocations. Our findings depart from the prevailing view that alternative end joining processes are generically translocation-prone.

DNA repair: Making the cut.

Analysis of the first step in repairing double-stranded-DNA breaks reveals that the Mre11 enzyme makes a DNA nick at a point separate from the break ends, creating an entry site for further processing by exonuclease enzymes. See Letter p.122 Homologous recombination at a DNA double-strand break uses a 3′-tailed molecule, which requires resection of the 5′ strand. Previous genetic analysis indicated that the MRX complex, consisting of the repair proteins Mre11, Rad50 and Xrs2, was required for resection. However its in vitro activity was puzzling, as only 3′ to 5′ resection was detected. Elda Cannavo and Petr Cejka have now resolved this conundrum. They find that in yeast, Sae2 nuclease activates MRX to make an initial endonucleolytic cut on the 5′ strand so that MRX can digest the 5′ strand back to the end in a 3′-to-5′ fashion. This activity is stimulated by blockage of the DNA end, which is consistent with in vivo situations such as meiosis, where the Spo11 complex remains bound after initiating a double-strand break.

Multifaceted role of the Topo IIIα-RMI1-RMI2 complex and DNA2 in the BLM-dependent pathway of DNA break end resection.

BLM, a RecQ family DNA helicase mutated in Bloom's Syndrome, participates in homologous recombination at two stages: 5′ DNA end resection and double Holliday junction dissolution. BLM exists in a complex with Topo IIIα, RMI1 and RMI2. Herein, we address the role of Topo IIIα and RMI1-RMI2 in resection using a reconstituted system with purified human proteins. We show that Topo IIIα stimulates DNA unwinding by BLM in a manner that is potentiated by RMI1-RMI2, and that the processivity of resection is reliant on the Topo IIIα–RMI1-RMI2 complex. Topo IIIα localizes to the ends of double-strand breaks, thus implicating it in the recruitment of resection factors. While the single-stranded DNA binding protein RPA plays a major role in imposing the 5′ to 3′ polarity of resection, Topo IIIα also makes a contribution in this regard. Moreover, we show that DNA2 stimulates the helicase activity of BLM. Our results thus uncover a multifaceted role of the Topo IIIα–RMI1-RMI2 ensemble and of DNA2 in the DNA resection reaction.

DNA2 Cooperates with the WRN and BLM RecQ Helicases to Mediate Long-range DNA End Resection in Human Cells

The 5'-3' resection of DNA ends is a prerequisite for the repair of DNA double strand breaks by homologous recombination, microhomology-mediated end joining, and single strand annealing. Recent studies in yeast have shown that, following initial DNA end processing by the Mre11-Rad50-Xrs2 complex and Sae2, the extension of resection tracts is mediated either by exonuclease 1 or by combined activities of the RecQ family DNA helicase Sgs1 and the helicase/endonuclease Dna2. Although human DNA2 has been shown to cooperate with the BLM helicase to catalyze the resection of DNA ends, it remains a matter of debate whether another human RecQ helicase, WRN, can substitute for BLM in DNA2-catalyzed resection. Here we present evidence that WRN and BLM act epistatically with DNA2 to promote the long-range resection of double strand break ends in human cells. Our biochemical experiments show that WRN and DNA2 interact physically and coordinate their enzymatic activities to mediate 5'-3' DNA end resection in a reaction dependent on RPA. In addition, we present in vitro and in vivo data suggesting that BLM promotes DNA end resection as part of the BLM-TOPOIIIα-RMI1-RMI2 complex. Our study provides new mechanistic insights into the process of DNA end resection in mammalian cells.