Increased proliferation of the tumor cells and deprivation of oxygen increases utilization of adenosine triphosphate (ATP) and activation of the cancer-associated ectoenzymes ectonucleoside triphosphate diphosphohydrolase-1 (CD39) and ecto-5′-nucleotidase (CD73), which catalyze sequential dephosphorylation of ATP to adenosine monophosphate (AMP) and extracellular adenosine [24,25], whereas cyclic ADP-ribose hydrolase (CD38) catalyzes the hydrolysis of nicotinamide adenine dinucleotide (NAD) [26]. This evidence concerns the gene ENTPD1 and cancer.