IDO1 and neoplasm: Several factors that affect the treatment response of immune checkpoints have been identified, and they include tumor antigens (such as tumor mutation load and microsatellite instability) (Wang et al., 2017) and immunosuppressive and inflammatory cells or proteins (such as TILs and tumor-related immune cells, gene markers, CTLA-4, IDO and PD-L1) (Cristescu et al., 2018).