Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Merkel Cell Carcinoma ?
    ### Response:
    Key Points
                    - Merkel cell carcinoma is a very rare disease in which malignant (cancer) cells form in the skin.    - Sun exposure and having a weak immune system can affect the risk of Merkel cell carcinoma.    - Merkel cell carcinoma usually appears as a single painless lump on sun-exposed skin.
    