Given the lack of improved therapeutic outcomes with PD1 and selective IDO1 inhibition among unselected patient populations in the recent phase 3 ECHO-301/KEYNOTE-252 trial21, our findings highlight the need and feasibility of patient stratification by monitoring serum Kyn/Trp alterations and more generally point to the relevance of metabolic adaptations in cancer immunotherapy. The gene discussed is PDCD1; the disease is cancer.