CD39, nucleoside triphosphate diphosphohydrolase-1 (NTPDase 1), is an ectoenzyme that degrades ATP to AMP. It is a member of ectonucleoside triphosphate dihydrolases (E-NTPDases) which are involved in regulation of extracellular nucleotide catabolism and controlling the extracellular nucleoside triphosphate pool (NTP). CD39 is the dominant member of this family in the immune system and involved in suppression of inflammation and control of platelet activation. CD39 is expressed on B cells, dendritic cells, and a subset of T cells including regulatory T cells and memory T cells. The coordinated expression of CD39/CD73 on Tregs and the adenosine A2A receptor on activated T effector cells generates immunosuppressive loops. In human studies, it has been reported that CD4+CD25-CD39+ T cells are T inducers.