Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to purine nucleoside phosphorylase deficiency ?
    ### Response:
    Purine nucleoside phosphorylase deficiency is caused by mutations in the PNP gene. The PNP gene provides instructions for making an enzyme called purine nucleoside phosphorylase. This enzyme is found throughout the body but is most active in specialized white blood cells called lymphocytes.
    