Skin cancer is a disease in which malignant (cancer) cells form in the tissues of the skin.

The skin is the body's largest organ. It protects against heat, sunlight, injury, and infection. Skin also helps control body temperature and stores water, fat, and vitamin D. The skin has several layers, but the two main layers are the epidermis (upper or outer layer) and the dermis (lower or inner layer). Skin cancer begins in the epidermis, which is made up of three kinds of cells:

• Squamous cells: Thin, flat cells that form the top layer of the epidermis.
• Basal cells: Round cells under the squamous cells.
• Melanocytes: Found in the lower part of the epidermis, these cells make melanin, the pigment that gives skin its natural color. When skin is exposed to the sun, melanocytes make more pigment, causing the skin to tan, or darken.

Skin cancer is the most common cancer in the United States.

The three most common types of skin cancer are basal cell carcinoma, squamous cell carcinoma, and melanoma. Basal cell carcinoma is the most common and melanoma is the least common skin cancer.

Most basal cell and squamous cell skin cancers can be cured, but people with these types of cancer have a higher risk for developing other skin cancers. Melanoma causes about three fourths of skin cancer deaths in the United States and is the focus of this screening summary.

Skin color and exposure to sunlight can affect the risk of developing melanoma.

Anything that increases your chance of getting a disease is called a risk factor. Risk factors for melanoma include the following:

• Having a fair complexion (blond or red hair, fair skin, freckles, blue eyes).
• Being exposed to natural sunlight or ultraviolet light (such as tanning booths) over long periods of time.
• Being white and older than 20 years.
• Having several large or many small moles.
• Having a family history of unusual moles (atypical nevus syndrome).
• Having a family or personal history of melanoma.

The best defense against skin cancer is protection from sunlight and ultraviolet light. Refer to the PDQ summary on Prevention of Skin Cancer for more information.