About 1500 cases of breast cancer and 400 breast cancer deaths occur in men each year in the United States. Men with a history of testicular or breast disease, a family history of breast cancer in women, or Jewish ancestry have a higher-than-average risk for developing breast cancer. Men with a genetic condition called Klinefelter syndrome have a much higher risk for breast cancer than do men without this condition. Abnormalities in the BRCA2 gene, which greatly increase the risk for breast cancer in women, seem to also increase the risk in men. In contrast, abnormalities in the BRCA1 gene, another gene related to breast cancer in women, do not seem to be associated with breast cancer in men. Breast enlargement in men (gynecomastia) does not seem to be a risk factor for breast cancer. In general, men develop the same kinds of breast cancer as do women. However, breast cancer in men is more likely than breast cancer in women to be hormone receptor positive. This means that the cancer cells depend on the hormones estrogen or progesterone for growth. Molecular markers of breast cancer (proteins that appear in abnormal forms or at higher levels in cancer cells) seem to be similar in women and in men. Men are more likely to have a delay between onset of symptoms and diagnosis, probably because many are unaware that breast cancer can occur in men. The existing evidence suggests that men with breast cancer should get the same treatments as do women. However, given the large proportion of hormone receptor–positive tumors in men, hormone-related treatments may prove to be particularly useful.