IDO1 and cancer: For example, programmed cell death 1 (PD-L1), cytotoxic T lymphocyte antigen 4 (CTLA4), and the indoleamine 2, 3-dioxygenase 1 (IDO1) have achieved impressive success in the treatment of different cancer types, especially the combined treatment of PD-L1 and CTLA4 [49].