An interesting study performed by Botticelli et al. shows that higher kynurenine/tryptophan (Kyn/Trp) ratio representing IDO1 activity could predict resistance to anti-PD-1 treatment in NSCLC, which suggest the possibility of using anti-PD-1 plus IDO inhibitors in those patients with a high level of Kyn/Trp ratio. This evidence concerns the gene IDO1 and non-small cell lung carcinoma.