CHEK2 and breast cancer: Previous studies reported that biallelic CHEK2 PVs are associated with higher rates of breast cancer and earlier age at onset (median age, 40.5-43.5 years vs 47-49 years for monoallelic).2,3,4,5 Biallelic CHEK2 PVs were also associated with an earlier age at onset of any cancer by a decade (median age, 37 years vs 47 years for monoallelic PVs).2