Melanoma is a type of skin cancer that originates in cells called melanocytes. The skin is the largest organ of the body, protecting the internal organs from injury and invasion from harmful substances. It is composed of three layers – the epidermis, the dermis and the subcutis. The top layer is called the epidermis.

Melanocytes, which protect the skin from the harmful rays of the sun, are located in the epidermis, which is about 1/100th of an inch (or 0.2 millimeters [mm]) thick. Melanocytes produce melanin, the pigment that gives skin its color. 

Within the epidermis layer are four sublayers. The outermost sublayer (stratum corneum) of the epidermis is composed of dead keratinocytes (also called squamous cells). Below this level are live keratinocytes, which produce an important protein that helps protect the body. The lowest sublayer is composed of basal cells.

Like all cancers, melanoma results from unrepaired damage to DNA, genetic material that is present in every cell. This damage causes the cells to multiply continuously until they form a growth or tumor (a mass of excess tissue), usually appearing as a mole on the skin or sometimes in the eye. At an early stage, melanoma is usually highly curable by simply excising (cutting out) the mole.

However, melanoma is capable of spreading to every tissue and organ in the body. Melanoma can spread outward on the surface of the skin or deep into the layers of the skin (reaching the lymphatic system and bloodstream).

While the vast majority of melanomas develop on the skin, they have also been found in the eye (intraocular melanoma).

Although melanoma can spread to any organ or tissue, the most common sites for metastasis are the lungs and liver.  It is also known to spread to the brain and spinal cord and to the bones.

Melanoma is strongly linked to excessive exposure to ultraviolet (UV) radiation, primarily from the sun. It is believed that intense, intermittent exposure to UV rays creates a greater risk for melanoma than prolonged low-grade exposure. People who work for long periods in the sun appear to be less at risk than those who get occasional but intense exposure to the sun, such as indoor workers who spend several hours in the sun on weekends.

Most cases occur in white people, and it most often appears on the trunks of fair-skinned men and the lower legs of fair-skinned women. People who have darker pigment are at lower risk but they also can develop melanoma.

Melanoma accounts for 4 percent of all skin cancers, but is responsible for about 75 percent of skin-cancer related deaths, according to the American Cancer Society (ACS). Since 1973, mortality has sharply increased by 50 percent, mostly among older white men. Although the number of new cases of melanoma has slowed among both men and women, the disease remains on the rise. The ACS estimates that there will be 62,190 new cases of melanoma in the United States in 2006 and nearly 8,000 people will die of the this cancer.

Melanoma is highly curable, with a five-year survival rate of 97 percent when caught at a localized stage (meaning it has not spread), according to the Centers for Disease Control and Prevention. The survival rate drops to 63 percent for melanoma that has spread to nearby areas and to 18 percent if the cancer has spread to distant organs.

Melanoma can be confused with other forms of skin cancer that are less dangerous.