Breast cancer is a malignant tumor that has developed from cells of the breast.
The disease occurs mostly in women, but does occur rarely in men. Breast cancer is the most common cancer among women, excluding skin cancers. The American Cancer Society estimates that in 1999 about 175,000 new cases of invasive breast cancer will be diagnosed among omen in the United States. An estimated 1,300 cases will be diagnosed among men. Breast cancer is the second leading cause of cancer death in women, exceeding lung cancer.