Half of all cases of melanoma, the most serious form of skin cancer, develops from moles. The average Caucasian American adult has 24 moles. A 1998 study in the Journal of the American Medical Association confirmed that the number, size and appearance of moles all affect the risk of melanoma. Any mole that changes shape, color or size; any sore that doesn’t heal; any persistent patch of irritated skin; or any new growth, may be a sign of cancer and require professional attention.