IDO1 and neoplasm: Cheng et al. showed that in tumor cells, ROS induced by mitochondrial Lon, a chaperone and DNA-binding protein involved in protein quality control and stress response pathways, triggers mtDNA damage, and Ox-mtDNA is released into the cytoplasm to activate IFN signaling, which further upregulates the expression of programmed death ligand-1 (PD-L1) and indoleamine 2,3-dioxygenase (IDO-1) to inhibit T-cells activation.