One possible explanation for the high incidence of contralateral breast cancer (CBC) in the BRCAx group is that there may be mutations in high-penetrance genes other than BRCA1/2, such as PTEN, CDH1, and CHEK2, or the presence of common low-penetrance variants that increase the risk of developing cancer in the contralateral breast. This evidence concerns the gene CHEK2 and breast carcinoma.