Skin cancer is a malignant growth on the skin which can have many causes. Skin cancer generally evelops in the epidermis (the outermost layer of skin), so a tumor is usually clearly visible. This makes most skin cancers detectable in the early stages. There are three common types of skin cancer, each of which is named after the type of skin cell from which it arises. Cancers caused by UV exposure may be prevented by avoiding exposure to sunlight or other UV sources, and wearing sun-protective clothes. The use of sunscreen had been recommended in the past, but there is an increasing body of evidence that sunscreen is not entirely safe.