Merkel cell carcinoma (MCC) is a rare, aggressive skin cancer. It usually develops as a single, painless, bump on sun-exposed skin. The bump may be skin-colored or red-violet, and tends to grow rapidly over weeks to months. It may spread quickly to surrounding tissues, nearby lymph nodes, or more distant parts of the body. Factors associated with developing MCC include increasing age, fair skin, a history of extensive sun exposure, chronic immune suppression, and the Merkel cell polyomavirus. This virus has been detected in about 80% of people with MCC. Treatment may include surgery, radiation therapy, and/or chemotherapy. Treatment options and prognosis depend on the location(s) and size of the cancer, whether it has just been diagnosed or has come back (recurred), and how deeply it has grown into the skin.

Cancer is caused by changes (mutations) in the DNA inside of cells. These mutations cause the cells to grow and divide into new cells, when they should not. The mutations that cause MCC are not inherited from a parent, but occur by chance during a person's lifetime (they are acquired, or somatic mutations). In many cases, it is not known what directly causes these mutations to occur. However, several factors are thought to increase the risk for mutations to occur - such as exposure to sunlight.

Merkel cell polyomavirus is frequently involved in the development of MCC and is present in about 80% of MCC tumors tested. While the majority of people have been exposed to this virus by adulthood, it appears that the virus does not cause any symptoms except in the very rare situations in which it leads to MCC.

Other risk factors that have been associated with MCC include:

• being older than age 50
• having fair skin
• having a history of extensive sun exposure
• having chronic immune suppression (e.g. organ transplantation or HIV)

Having one or more risk factors does not mean that a person will develop MCC. Most people with risk factors will not develop MCC.

MCC does not seem to run in families. While DNA changes (mutations) found in the cells of MCC tumors can lead to MCC, these types of mutations are not inherited from a person's parents. They are referred to as somatic mutations and occur during a person's lifetime, often as random events.