BRCA1 encodes an E3 ubiquitin-protein ligase that plays a central role in DNA repair and maintaining genomic stability. The protein specifically mediates formation of 'Lys-6'-linked polyubiquitin chains and functions as part of the BRCA1-BARD1 heterodimer to coordinate cellular pathways including DNA damage repair, ubiquitination, and transcriptional regulation. BRCA1 is essential for homologous recombination repair through direct interaction with PALB2, which facilitates loading of BRCA2-RAD51 repair machinery at DNA breaks. The protein regulates cell cycle checkpoints, particularly G2/M transitions following DNA damage, and is required for appropriate S-phase and G2-phase arrests after ionizing radiation. Beyond DNA repair, BRCA1 regulates centrosomal microtubule nucleation, inhibits lipid synthesis by binding phosphorylated ACACA, and acts as a transcriptional activator. Germline pathogenic variants in BRCA1 cause hereditary breast and ovarian cancer syndrome, with lifetime breast cancer risk reaching 80% and ovarian cancer risk approximately 40% 1. BRCA1-associated breast cancers typically present at younger ages and exhibit increased gene expression variability compared to non-BRCA1 tumors 2. MRI surveillance significantly reduces breast cancer mortality in women with BRCA1 variants 3. Population-based BRCA testing proves cost-effective compared to family history-based testing, potentially preventing thousands of cancers 4. At the population level, gnomAD v4.1 classifies this gene as LoF-tolerant because heterozygous carriers are viable; this is distinct from clinical pathogenicity in disease contexts.