As a dominant driver of tumor immune escape, immune checkpoints, such as programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1), cytotoxic T-lymphocyte-associated protein 4 (CTLA4) and indoleamine 2,3-dioxygenase (IDO), have long become a topic of great interest in biomedical field 25-27. Here, IDO1 is linked to neoplasm.