Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) thiopurine S-methyltransferase deficiency ?
    ### Response:
    Thiopurine S-methyltransferase (TPMT) deficiency is a condition characterized by significantly reduced activity of an enzyme that helps the body process drugs called thiopurines. These drugs, which include 6-thioguanine, 6-mercaptopurine, and azathioprine, inhibit (suppress) the body's immune system.
    