Stage 0 melanoma is caused by the overproduction of certain skin cells called melanocytes, which are the cells that produce skin pigment. This cell overgrowth usually starts on an existing mole on the skin or as a new skin growth. Generally, these growths are found in areas of skin that are exposed to sunlight, although they may be found in other parts of the body, including internal mucous membranes like those found in the esophagus and urinary tract. Stage 0 melanoma is the very first stage of skin cancer, so growths usually affect only the epidermis, or topmost layer of skin, and spread across the skin's surface. If left untreated, stage 0 melanoma can spread rapidly to other parts of the body.