ENTPD1 and neoplasm: Wildly expressed on vascular endothelial cells, immune cells and tumor cells, CD39 (ecto-nucleoside triphosphate diphosphohydrolase-1) is the dominant and rate-limiting ectonucleotidase that hydrolyzes ATP and adenosine diphosphate (ADP) to adenosine monophosphate (AMP) (66).