PNP, encoded by 6 exons on human chromosome 14q13, is a homotrimetic enzyme of approximately 96 k dalton. This enzyme reversibly catalyzes the phosphorolysis of purine nucleoside to their respective purine bases and the corresponding pentose-1-phosphate. The ultimate degradative product of purine nucleoside is uric acid, which is a depleted product with the absence of PNP activity. This leads to accumulation of deoxy-guanosine triphosphate, which inhibits the enzyme ribonucleoside reductase resulting in DNA synthesis block and T cell proliferation. Basic and brief knowledge of PNP biochemistry and functional physiology is summarized for the understanding of clinical features of PNP deficiency.