Purine nucleoside phosphorylase (PNP) is an enzyme which reversibly catalyzes the phosphorolysis of purine nucleosides. The PNP 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 [taken from NCBI Entrez Gene (Gene ID: 4860)].