IDO1 and neoplasm: Despite considerable advancements, the response rate to ICIs is currently limited to 10%–25% in most tumour types (Schoenfeld and Hellmann, 2020), and those with deficient immunogenic epitopes (low mutational burden) (Verdegaal et al., 2016; Tran et al., 2017), impoverished tumour-infiltrating lymphocytes (Galluzzi et al., 2018; Fanale et al., 2022), or profuse immunosuppressive factors (such as PD-L1, CD73, and indoleamine 2,3-dioxygenase 1) (Young et al., 2016; Chen and Mellman, 2017; Song et al., 2021) are less likely to respond (Galluzzi et al., 2018).