In the current study we investigated the role and mechanism of IDO induction in two murine models of influenza A infection using strain PR8 and the less virulent strain X31 by monitoring IDO induction in lungs and draining LNs and evaluating the consequences of ablating IDO1 genes, one of two homologous genes (IDO1 and IDO2) that encode IDO [20], [21], on the course of host primary and recall T cell responses to influenza infection. This evidence concerns the gene IDO1 and influenza.