This gene encodes an enzyme which reversibly catalyzes the phosphorolysis of purine nucleosides. This enzyme, together with adenosine deaminase (ADA) serves a key role in purine catabolism. The enzyme is trimeric, containing three identical subunits. Mutations which result in nucleoside phosphorylase deficiency result in defective T-cell (cell-mediated) immunity but can also affect B-cell immunity and antibody responses. Neurologic disorders may also be apparent in patients with immune defects. A known polymorphism at aa position 51 that does not affect enzyme activity has been described. A pseudogene has been identified on chromosome 2.