Genes control the functions inside a cell. Special genes control how cells divide and grow. In cancer cells, these genes are damaged. Cancer occurs when the damaged genes stimulate the cell to multiply out of control. The exact cause for this damage is unknown. Damaged genes can be inherited: the risk of cancer is higher if other family members have had cancer.

Cancer cells can develop from any tissues inside the breast: mild glands, mild ducts, fatty tissue or nipple. The cells can spread through the lymphatic system or the bloodstream. Breast cancer spreads most commonly to the bones, lungs, brain and liver.