ENPP1 and neoplasm: In the extracellular environment, ATP can be hydrolyzed into AMP by ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) and CD39 (ectonucleoside triphosphate diphosphohydrolase-1, NTPDase-1); in turn, AMP is dephosphorylated into adenosine by CD73 (ecto-5′-nucleotidase, Ecto5′NTase), a key regulator in some tumor processes such as invasion, migration, and metastasis [21,22].