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. This evidence concerns the gene BRCA1 and breast cancer.