The PNP gene gives instruction for making an enzyme that catalyzes the phosphorolytic breakdown of the N-glycosidic bond in the beta-(deoxy)ribonucleoside molecules. This enzyme contains three identical subunits and is found throughout the body but is most active in the lymphocytes. Defects in this enzyme are the cause of purine nucleoside phosphorylase deficiency, a disorder characterized by profound T cell deficiency, recurrent and opportunistic infections, progressive neurologic manifestations, autoimmunity disorders and malignancy.