IDO1 and neoplasm: KYN is a host‐derived metabolite that contributes to tumor progression, immune evasion, and Treg cell development.[11, 45] The conversion of TRP to KYN is regulated by IDO1 and the KYN/TRP ratio serves as a prognostic indicator for the response to ICB therapy.[46] Generally, more than 95% of dietary tryptophan is metabolized via the KYNurenine pathway.