Another recent study also has discovered a new mechanism of how a gut microbiome-derived tryptophan metabolite called I3A increases antitumor immunity via activation of AHR in CD8+ T cells and has synergy with inhibition of immune checkpoints.5 Taken together, tryptophan catabolites of the gut microbiota deeply influence tumor growth in a variety of ways, which may depend upon types of cancer and therapy and the composition of the intestinal and tumor microbiome. This evidence concerns the gene AHR and cancer.