Immune escape is a crucial feature of tumor, in which tumor cells establish multiple mechanisms to evade host immune surveillance utilizing its heterogeneity and genetic variability, such as the expression of inhibitory receptors programmed cell death-ligand 1 (PD-L1), cytotoxic T lymphocyte-associated antigen 4 and indoleamine 2,3-dioxygenase (IDO) [1–3]. Here, IDO1 is linked to neoplasm.