The rates of contralateral breast cancer among BRCA1 and BRCA2 variant carriers were significantly higher than that among noncarriers (BRCA1 vs noncarriers, 13.9% [26 of 187] vs 2.5% [198 of 7905]; P < .001; BRCA2 vs noncarriers, 13.5% [41 of 304] vs 2.5% [198 of 7905]; P < .001) (eTable 7 in the Supplement). This evidence concerns the gene BRCA1 and breast cancer.