Melanoma begins in the skin cells (melanocytes) that make the pigment that colors your skin. It is a serious form of skin cancer that usually starts in unmarked, normal skin. But it can start in a mole or birthmark.
When melanocytes grow out of control, they can spread from the epidermis, which is the upper layer of skin, down into the dermis. If melanoma isn't treated, it can spread to other parts of your body.
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