Most appear on the skin during the first 20 years of life, especially during puberty. They are caused by an overproduction of melanin in clumps of skin cells. The cause of this melanin overproduction is not known. At first moles are flat, but they may become raised. Many grow larger and some develop hairs.
Most moles are harmless.
Some moles, however, may be considered unsightly because of their appearance or location. Malignant melanoma is the least common but most dangerous form of skin cancer. Research suggests that malignant melanoma in adults may be related to sunburn in childhood. Because most moles are harmless, they usually need no treatment. Some may be removed for cosmetic reasons.