A woman's breast is made up of milk glands and milk ducts, surrounded by fatty tissue and connective supports. Uncontrolled growth of cells in any of these breast tissues can cause breast cancer.

Breast cancer is the most common cancer among women. It tends to occur at an earlier age than do other cancers or heart attacks and strokes. Chances of developing breast cancer rise dramatically as women age: at age 25, the chances of getting the disease are less than 1 in 1,000. At age 50, the incidence has gone up to 1 in 63 women, and at age 75, it's 1 in 15. Breast cancer is rare in men, accounting for less than 1% of all cases.