CHEK2 and breast cancer: Studies employing gene panel sequencing in a large numbers of familial breast cancer cases suggest that approximately 4% carry a pathogenic or likely pathogenic variant in a breast cancer gene other than BRCA1 or BRCA2. 43, 44, 45 We found two index cases carrying the CHEK2*1100delC pathogenic variant (consistent with high frequency of this variant in the Dutch population), and four possibly pathogenic variants in other susceptibility genes.