ADA and Autoimmunity: It is also associated with neurologic dysfunction in about two thirds of patients and autoimmunity in about one third.1–3 PNP deficiency was first described by Giblett et al in 1975 shortly after their discovery of a deficiency of another enzyme of the purine metabolic pathway, adenosine deaminase (ADA).4 PNP catalyzes the reversible degradation of inosine and deoxyinosine to hypoxanthine and guanosine, and deoxyguanosine to guanine.