Negative immune checkpoints including cytotoxic T lymphocyte antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1), and indoleamine 2,3-dioxygenase (IDO), etc. inhibit the activity of T cells to kill tumor cells and promote tumor cells the escape 97. This evidence concerns the gene IDO2 and neoplasm.