IDO1 and neoplasm: Indoleamine 2,3-dioxygenase (IDO) is a human extrahepatic tryptophan metabolism rate-limiting enzyme that mediates tryptophan (TRP) depletion and limits TRP availability in tumor cells and innate immune cells, thus triggering an effector pathway that interferes with cytotoxic T cell activation while inducing proliferative activation of regulatory T cells (Tregs) (Löb et al., 2009).