Skin cancer. Gross specimen of a malignant melanoma (dark area), a type of skin cancer. This cancer arises from the skin's melanocytes, the cells that produce the pigment (melanin) that give skin its colour. The main cause of melanoma is exposure to ultraviolet radiation in sunlight. It is an aggressive cancer that often spreads (metastasises) to other parts of the body. Treatment is with surgical removal of the tumour, which can completely cure small tumours. Larger tumours are also treated with chemotherapy and/or immunotherapy. However, once the cancer has spread the prognosis is poor.