IDO1 and neoplasm: Though IDO expression is not required for immune tolerance under homeostatic conditions, it plays a critical role during acquisition of tumor-immune tolerance as indicated by studies demonstrating that IDOKO mice have increased tumor infiltration of effector T lymphocytes, decreased immunosuppressive immune cells, as well as increased survival (102–105), implicating non-redundant functions between IDO and TDO in the various cancer settings.