What is breast cancer? Cells normally develop, grow, multiply, and die in all tissues, including the breast. Breast cancer occurs when cells divide out of control and the mechanism that usually regulates cell division is not working. In the most common types of breast cancer, it is thought that cancer cells begin either in the ducts or the lobular tissue of the breast. Cancer cells multiply and form a lump, or mass, called a tumor. The cells can also invade and destroy healthy tissue. Cells from a cancerous tumor in the breast can break away and spread to other organs in the body, a process known as metastasis.