Malignant melanoma. Light micrograph (LM) of a section through human skin showing melanoma cancer cells (brown). This cancer arises from the skin's melanocytes, the cells that produce the pigment (melanin) that give skin its colour. Their cell nuclei (purple) are extremely varied in shape, which is characteristic of cancer cells. 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. Magnification: x 100 when printed 10cm wide.