The mutations of BRCA1, BRCA2 and MMR gene may play very important roles in the onset of the male breast cancer; and other genetic factors involved could include AR gene, CHEK2 gene, cytochrome P45017 (CYP17), the XXY karyotype (Klinefelter syndrome), and the PTEN tumor suppressor gene associated with Cowden syndrome, and so on. The gene discussed is BRCA1; the disease is Klinefelter syndrome.