ESR1 and breast cancer: Among all ER + BC tumors, 8.8% (N = 71/805; N = 60 ER + HER2-, N = 10 ER + HER+, and N = 1 ER + HER2 status unknown) were BRCA1/2-deficient; of those, 40.8% (N = 29/71) had a pathogenic mutation in BRCA1, and 59.2% (N = 42/71) had a pathogenic mutation in BRCA2 (see Supplemental Fig. 1 for GIS distributions of BRCA1-deficient and BRCA2-deficient samples).