IDO1 and neoplasm: Besides being unable to degrade tryptophan, the apo-IDO1 assumes a conformation prone for transducing an intracellular signal, as previously demonstrated in tumor cells ectopically expressing a loss-of-function mutant of IDO1 (namely, the apo-form) as well as in response to the IDO1 catalytic inhibitor epacadostat (13, 14, 18).