Melanoma is predominantly associated with excessive sun exposure, but can also arise in areas of the body protected from sunlight. Like all skin cancers, malignant melanoma is most common in fair-complexioned individuals who have a history of bad sunburns and chronic sun exposure. It also tends to run in some families with a recognized gene mutation, which seems to confer increased susceptibility to the disease.
For unknown reasons, a small percentage of melanomas occur on areas that are normally protected from sunlight, such as the palms of the hands, the soles of the feet, or under the nails.

Melanocytes and moles

Melanocytes produce melanin, the pigment that gives skin its natural color. When skin is exposed to the sun, melanocytes produce more pigment, causing the skin to tan, or darken.
Sometimes, clusters of melanocytes and surrounding tissue form noncancerous growths called moles. (Doctors also call a mole a nevus; the plural is nevi.) Moles are very common. Most people have between 10 and 40 moles. Moles may be pink, tan, brown, or a color that is very close to the person's normal skin tone. People who have dark skin tend to have dark moles. Moles can be flat or raised. They are usually round or oval and smaller than a pencil eraser. They may be present at birth or may appear later on -- usually before age 40. They tend to fade away in older people. When moles are surgically removed, they normally do not return.