The enzyme purine nucleoside phosphorylase (PNP) catalyses the reversible cleavage of purine nucleosides to the purine base and ribose-1-phosphate. Several cases of a rare genetic disorder in which PNP is lacking have been reported in children. These children were found to be T-cell immunodeficient while their B-cell immunity remained normal. This observation helped establish the relationship between PNP and T-cells and provided the impetus for the development of inhibitors of PNP which may be useful for the treatment of T-cell proliferative disorders. Proliferating T-cells have been implicated in cutaneous T-cell lymphoma and in T-cell leukaemias. Autoimmune diseases such as psoriasis, rheumatoid arthritis, systemic lupus erythematosus, Crohn’s disease and Type I diabetes are also thought to be T-cell mediated. Other diseases which may benefit from treatment with PNP inhibitors are gout and infection from human immunodeficiency virus (HIV). The half-life of nucleoside antivirals may also be increased by ...