Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Mixed connective tissue disease ?
    ### Response:
    Mixed connective tissue disease (MCTD) is a rare autoimmune disorder that is characterized by features commonly seen in three different connective tissue disorders: systemic lupus erythematosus, scleroderma, and polymyositis. Some affected people may also have symptoms of rheumatoid arthritis.
    