Moreover, IFN-γ, a known inducer of IDO1, can also induce IDO2 expression in cancer cell lines.65,68 Because PD-1 and PD-L1 blockade leads to an increase in cytotoxic TILs, which in turn secrete pro-inflammatory molecules, such as IFN-γ,164,165 IDO2 expression could be induced in the tumour and its activity might compensate for IDO1 inhibition (Fig. 2c). This evidence concerns the gene IDO1 and cancer.