Most breast cancers in men are ductal carcinomas. Ductal means the cancer started in the milk pipes of the breast, called ducts. These cancers are usually invasive because they start inside the duct and then break through the wall of the duct, growing into the normal surrounding breast tissue.
Mutations in the breast cancer susceptibility genes BRCA1, BRCA2, and CHEK2 are known risk factors for female breast cancer. Mutations in BRCA1 and BRCA2 also are associated with male breast cancer (MBC). Similarly, it had been suggested in the original CHEK2 identification report that the CHEK2 1100delC mutation confers an increased risk for MBC