Hypoxanthine, also known as purine-6-ol or Hyp, belongs to the class of organic compounds known as purines. Purines are a bicyclic aromatic compound made up of a pyrimidine ring fused to an imidazole ring. Hypoxanthine is also classified as an oxopurine, Hypoxanthine is a naturally occurring purine derivative and a reaction intermediate in the metabolism of adenosine and in the formation of nucleic acids by the nucleotide salvage pathway. Hypoxanthine exists in all living species, ranging from bacteria to plants to humans. Hypoxanthine has been detected, but not quantified in, several different foods, such as radish (var.), mountain yams, welsh onions, greenthread tea, and common beets. Hypoxanthine is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Biologically, hypoxanthine can be formed a number of ways. For instance, it is one of the products of the action of xanthine oxidase on xanthine. However, more frequently xanthine is formed from oxidation of hypoxanthine by xanthine oxidoreductase. The enzyme hypoxanthine-guanine phosphoribosyltransferase converts hypoxanthine into IMP in the nucleotide salvage pathway. Hypoxanthine is also a spontaneous deamination product of adenine. Under normal circumstances hypoxanthine is readily converted to uric acid. In this process, hypoxanthine is first oxidized to xanthine, which is further oxidized to uric acid by xanthine oxidase. Molecular oxygen, the oxidant in both reactions, is reduced to H2O2 and other reactive oxygen species. In humans, uric acid is the final product of purine degradation and is excreted in the urine. Within humans, hypoxanthine participates in a number of other enzymatic reactions. In particular, hypoxanthine and ribose 1-phosphate can be biosynthesized from inosine through its interaction with the enzyme purine nucleoside phosphorylase. Hypoxanthine is also involved in the metabolic disorder called the purine nucleoside phosphorylase deficiency. Purine nucleoside phosphorylase (PNP) deficiency is a disorder of the immune system (primary immunodeficiency) characterized by recurrent infections, neurologic symptoms, and autoimmune disorders. PNP deficiency causes a shortage of white blood cells, called T-cells, that help fight infection. Affected individuals develop neurologic symptoms, such as stiff or rigid muscles (spasticity), uncoordinated movements (ataxia), developmental delay, and intellectual disability. PNP deficiency is associated with an increased risk to develop autoimmune disorders, such as autoimmune hemolytic anemia, idiopathic thrombocytopenic purpura (ITP), autoimmune neutropenia, thyroiditis, and lupus.