The contribution of BRCA1 and BRCA2 for inherited cases of breast cancer was assessed in 237 families, each with at least 4 cases of breast cancer. The disease was associated with BRCA1 in 52% of families, BRCA2 in 32% of families, and neither of these in 16% of families. Penetrance estimates were also provided in the study, which can be used for better counselling of mutation carriers. This evidence concerns the gene BRCA1 and breast carcinoma.