Interestingly, IDO1 catalyzes the initial and rate limiting step in conversion of L-tryptophan into kynurenine (Hayaishi, 1976), is upregulated during HIV-1 infection or IFN-γ stimulation (Favre et al., 2010) and can block the replication of HIV-1 and other viruses through the inhibition of viral protein production by L-tryptophan depletion (Kane et al., 2016; Mao et al., 2011; Obojes et al., 2005). This evidence concerns the gene IFNG and HIV-1 infection.