ENTPD1 and neoplasm: However, it is quite challenging to maintain eATP at high levels in the tumor microenvironment for long periods because of the abundant expression of ATP-degrading enzymes chiefly CD39/ENTPD1 (nucleoside triphosphate diphosphohydrolase-1) by tumor-associated vascular, immune and stromal cells, and/or tumor cells per se [2], [21].