Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to thiopurine S-methyltransferase deficiency ?
    ### Response:
    TPMT deficiency results from changes in the TPMT gene. This gene provides instructions for making the TPMT enzyme, which plays a critical role in breaking down (metabolizing) thiopurine drugs. Once inside the body, these drugs are converted to toxic compounds that kill immune system cells in the bone marrow.
    