Actinic keratoses on the arm of a 61 year old white male patient following two weeks of treatment with fluorouracil. Actinic keratoses are precancerous skin growths caused by sun damage. The damage occurs years before the lesion appears. Actinic, or solar, keratoses are mostly found on areas of skin that are exposed to the sun most often, including the backs of hands and the face. It is important to remove the lesions as they can develop into skin cancer. Fluorouracil, marketed under the name Efudix, is a thymidylate synthase inhibitor and cytotoxic (toxic to cells) drug.