On one hand, IFN-γ is the most prominent inducer of the IDO1 gene [71]; on the other hand, although several studies demonstrate that IFN-γ could induce the expression of IDO2 in human mesenchymal stem cells and some cancer cells [72,73], in human dendritic cells, the cytokine is ineffective [74]. The gene discussed is IDO2; the disease is cancer.