Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) epidermal nevus ?
    ### Response:
    An epidermal nevus (plural: nevi) is an abnormal, noncancerous (benign) patch of skin caused by an overgrowth of skin cells. Epidermal nevi are typically seen at birth or develop in early childhood. They can be flat, tan patches of skin or raised, velvety patches.
    