NT5E and neoplasm: Moreover, the fact that NR supplementation restored intracellular NAD +content in CD73-deficient tumor cells, and that CD73 failed to increase NAD +levels in the absence of NRK1 (since NRK1 is necessary and rate-limiting for the generation of NAD +from exogenous NR Ratajczak et al., 2016), further support the notion that tumor cell-associated CD73 contributes to maintain intracellular NAD pools.