Breast cancer occurs when breast cells start growing out of control. These cells form a tumor and can be seen as a lump in the breast. Breast cancer occurs both in men and women, although HER2 breast cancer is common only amongst women. It can start growing in the milk ducts (ductal cancers) or in the glands that produce milk (lobular cancers). Breast cancer is mostly detected after symptoms start to appear. Somet. . .