Purine Nucleoside Phosphorylase (PNP) catalyzes the phophorolysis of N-ribosidic bonds of purine nucleosides and deoxynucleosides. Physiological substrates of PNP include inosine, guanosine, and 2'-deoxyguanosine, but not adenosine. PNP is expressed in most tissues, with markedly greater expression in lymphoid tissues. Genetic deficiencies of PNP result in severely compromised T-lymphocyte function and neurologic dysfunction. PNP is used in assays for the measurement of inorganic phosphate.