CHEK2 and breast cancer: Biallelic CHEK2 PV carriers were significantly more likely to be diagnosed with breast cancer at or before age 50 (61.3%, 19/31), as well as to have a second breast cancer diagnosis (22.6%, 7/31), as compared to monoallelic carriers (23.9%, 1548/6473; p < 0.0001 and 8.1%, 523/6473; p = 0.0107, respectively).