The A1 monoclonal antibody specifically recognizes human CD39 which is also known as Ecto-ATP diphosphohydrolase 1 (Ecto-ATPase 1 or Ecto-ATPDase 1), Ecto-apyrase or NTPDase 1. CD39 is a ~78 kDa integral membrane glycoprotein that is encoded by ENTPD1 (Ectonucleoside triphosphate diphosphohydrolase 1). CD39 contains two transmembrane domains, one having a N- and the other a C-terminal cytoplasmic tail, and a large extracellular domain that has the enzymatic site. CD39 is also known as Lymphoid cell activation antigen because its expression is induced upon activation of T and B cells. CD39 is variably expressed on some regulatory T cells, NK cells, granulocytes, monocytes, dendritic cells, Langerhans cells, endothelial cells, platelets, and neurons. CD39 is a member of the ectonucleoside triphosphate dihydrolases (E-NTPDases) family that is involved in the regulation of extracellular nucleotide catabolism by controlling the extracellular nucleoside triphosphate pool (NTP). It functions as an ectoenzyme that can hydrolyze both nucleoside triphosphates and diphosphates such as ATP and ADP and thereby suppress inflammation and regulate platelet activation as well as purinergic neurotransmission. The ectoenzymes CD39 and CD73 can act in tandem to enable regulatory T cells (Treg) to generate immunosuppressive adenosine and thereby regulate immune responses.