Dermatoscope image of a melanoma in situ behind the ear of a 66 year old white male patient. Melanoma is a form of skin cancer that arises from melanocytes, the cells that produce melanin, the pigment that gives skin its colour. This is a melanoma in situ, an early form of the cancer confined to the outer most layer of cells in the skin. The main cause of melanoma is exposure to ultraviolet radiation in sunlight. It is an aggressive cancer that often spreads to other parts of the body. If this occurs, the prognosis is poor. Treatment is with surgical excision of the melanoma.