BRCA1 and breast cancer: After adjusting for genetic associations with BMI, AAM per-year on BC risk were HR = 0.90 (95%CI:0.83-0.98, p = 0.01) and HR = 0.95 (95%CI:0.86-1.04, p = 0.26) for BRCA1 and BRCA2 carriers, respectively, consistent with a protective effect of later AAM.<h4>Discussion</h4>MR analyses support causal associations between ANM and BC risk in BRCA2, but not BRCA1, and between AAM and BC risk in BRCA1 and BRCA2 PV carriers.