BRCA1 and breast carcinoma: Additionally, co-existing BRCA1 with CHEK2 cases was reported in analysis of a large Polish cohort (7,782 breast cancer and 6,233 control cases), in which patients were assessed for seven founder pathogenic variants in BRCA2 and CHEK2. Double heterozygosity was detected in 15 patients in the breast cancer group and 2 patients in the control group [36].