The rates of contralateral breast cancer among BRCA1 variant carriers were 13.7% for those receiving BCT (10 of 73), 20.0% for those receiving mastectomy with radiotherapy (6 of 30), and 11.9% for those receiving mastectomy alone (10 of 84), whereas the rate of contralateral breast cancer was significantly higher among BRCA2 variant carriers treated with mastectomy alone than among those treated with BCT (19.8% [26 of 131] vs 6.6% [7 of 106]; P = .003) and was nonsignificantly higher than among those treated with mastectomy with radiotherapy (19.8% [26 of 131] vs 11.9% [8 of 67]; P = .16). Here, BRCA1 is linked to breast cancer.