Familial breast cancer: A number of factors have been identified that increase the risk of breast cancer. One of the strongest of these risk factors is the history of breast cancer in a relative. About 15% to 20% of women with breast cancer have such a family history of the disease, clearly reflecting the participation of inherited (genetic) components in the development of some breast cancers. The breast cancer susceptibility genes BRCA1 and BRCA2 appear responsible for about 5% to 10% of all breast cancers. See related entries: Breast cancer susceptibility genes; BRCA1; BRCA2.