The abnormal development of cells lining, the lobules or ducts of the breast, causes breast cancer. Breast cancer is caused by a single altered cell or a clone. It usually takes a long time for such clonal cells to become fully malignant, invade, and spread. During the early stages of breast cancer, it is normally painless, although growths can stretch the skin and breast tissue, causing discomfort or pain. The pain may occur anywhere in the breast or in the nipple.

There are many defined forms of breast cancer ba...