Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) breast cancer ?
    ### Response:
    Breast cancer is a disease in which certain cells in the breast become abnormal and multiply uncontrollably to form a tumor. Although breast cancer is much more common in women, this form of cancer can also develop in men. In both women and men, the most common form of breast cancer begins in cells lining the milk ducts (ductal cancer).
    