Skin cancers can appear on normal skin without any warning, as well as develop on pre-existing moles. Therefore, it is important to carefully monitor moles. The source of the life-threatening skin cancer known as melanoma is the cells called melanocytes. Melanocytes produce the melanin pigment that gives color to the skin and enables tanning. Melanoma occurs when melanocytes become abnormal, leading to excessive and uncontrolled growth, and it has the potential to spread to other organs.