Melanoma is the most serious type of skin cancer. Skin has pigment cells called melanocytes. Most people have between 10 and 40 moles. Moles form when melanocytes grow together in clusters. Melanoma is more dangerous, but less common, than basal cell cancer and squamous cell cancer. These are the two most common types of skin cancer. Often, the first sign of melanoma is a change in a mole. Most melanomas have a black or black-blue area. Melanoma can also appear as a new, abnormal mole. Some unusual moles, called dysplastic nevi, are more likely than ordinary moles to turn into melanoma. Doctors use the letters ABCDE to describe features of a mole. The letters stand for asymmetry, border, color, diameter, and evolving. Tanning outdoors or in a tanning bed exposes skin to rays that raise the risk of melanoma. Your care team can check any mole that looks unusual or keeps changing.