Abstract An estimated 5-10% of breast cancer cases carry germline mutations in genes associated with a moderate to strong breast cancer risk, like BRCA1 and BRCA2. These mutations can influence disease onset, progression, biology of the tumor, potential for therapy, risk of recurrence and cancer risk of close relatives. This warrants clinical germline screening for such gene variants according to national guidelines on patient selection. In this study we aimed to molecularly and clinicopathologically contrast clinically screened patients and their tumors against unscreened patients. For this we used a population-based breast cancer cohort from southern Sweden with 6660 patients (14% of which had been screened) with accompanying RNA-sequencing and clinicopathological registry data. Screening subsets were analyzed within five clinically relevant breast cancer subgroups based on hormone receptor and lymph node status. Among other features, we investigated clinicopathological characteristics, risk of tumor recurrence, survival outcomes, tumor proliferation markers, immune cell content, genes differentially expressed and the pathways in which they were involved. Most differences were found in the triple negative breast cancer (TNBC) subgroup: when compared to patients that had not been screened, the screened subset contained younger individuals showing better overall survival, more proliferative tumors with upregulated genes that belong to DNA replication and double-strand break repair processes, lower proportion of M2 macrophages and higher proportion of T-helper 2 cells. As so, Swedish screening guidelines divide TNBC cases into two separate subsets possibly by selecting distinct underlying biological and mutational mechanisms associated with DNA repair deficiency (e.g., through BRCA1/2-deficiency), a disparity not seen in other breast cancer clinical subgroups. Understanding the molecular landscape of defined screening populations may improve future guideline updates, which in turn would also improve patient and resource handling. Citation Format: Deborah F. Nacer de Oliveira, Johan Vallon-Christersson, Hans Ehrencrona, Anders Kvist, Åke Borg, Johan Staaf. Molecular characteristics of breast cancer patients subjected to screening for germline predisposition in a population-based observational study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1434.