Breast cancer (malignant neoplasm of the breast) is a cancer that originates in breast tissue, most often in the inner lining of the milk ducts or lobules that supply milk to the ducts. It is a disease characterised by the uncontrolled growth of breast cells. It is the most common and deadliest cancer in women. Worldwide, cancer is the second most common cause of death, with almost 10 million women dying each year. Around half of all breast cancers occur in women with no risk factors. These cancers originating in the ducts are called ductal carcinomas; those originating in the lobules are called lobular carcinomas. Breast cancer is a disease that affects humans and other mammals. 

Although the overwhelming majority of cases in men involve women, men can also develop breast cancer.