Breast cancer is a malignant form of cancer that develops in breast tissue.
The most common type of breast cancer begins in the lining of the ducts and is called ductal carcinoma. Another type of breast cancer, called lobular carcinoma, arises in the lobules. The cause of most types of breast cancer is unknown. Breast cancer has been related to two genes, BRC1 and BRC2. Breast cancer occurs in men as well as women, but is much more common in women.