Skin cancer, which is the most common form of cancer in the United States, is the result of the abnormal growth of skin cells. Cancer can affect skin anywhere on the body, but most frequently appears on skin that is exposed to the sun. There are more than a million new cases of skin cancer in the United States each year.
Every day, skin cells die and new ones form to replace them in a process controlled by DNA. Skin cancer can form when this process does not work properly because of damage to DNA. New cells may form when they are not needed, or older cells may not die, both of which can cause a growth of tissue known as a tumor. DNA damage is often a result of ultraviolet radiation from sunlight or tanning lamps. In some cases, skin cancer affects areas of the skin that have not been exposed to the sun. Certain factors, such as fair skin, moles, a weakened immune system, heredity and age, also increase the risk of skin cancer.