Breast cancer is a disease in which cells in the breast grow out of control. There are different types of breast cancer. The type of breast cancer depends on which cells in the breast become cancerous. Breast cancer can start in different parts of the breast. The breast is made up of three main parts: lobules, ducts and connective tissue. Lobules are the glands that produce milk. The ducts are the tubes that carry milk to the breasts. Connective tissue (made up of fibrous and fatty tissue) surrounds and holds everything together. Most breast cancers start in the ducts or lobules.