Ectonucleoside triphosphate diphosphohydrolase-1 also known as CD39 or NTPDase-1, ENTPD1 was originally described as a B lymphocyte cell surface marker, but it is also present on the surface of natural killer cells, T cells, and some endothelial cells. ENTPD1 hydrolyzes the beta- and gamma phosphate residues of nucleotides, preferring ATP as the substrate. Through its hydrolysis of extracellular nucleotides, ENTPD1 plays a role in the regulation of purinergic signaling. ENTPD1 is also involved in the processes of thromboregulation and vascular inflammation. The amino acid sequence of mouse ENTPD1 is 77% and 72% identical to that of human and canine.