Skin cancer. Light micrograph of a section through a malignant melanoma, showing red blood cells (erythrocytes, red), connective tissue and blood vessel walls (purple) and macrophages (blue), which contain the skin's pigment melanin (dark blue). This cancer arises from the skin's melanocytes, the cells that produce the melanin. The main cause of melanoma is exposure to ultraviolet radiation in sunlight. It is an aggressive cancer that often spreads (metastasises) to other tissues of the body. Treatment is with surgical removal of the tumour, often combined with chemotherapy and/or immunotherapy. However, once the cancer has spread, the prognosis is poor.