IDO1 and neoplasm: Despite the inherent difficulty of detecting labeling subsequent to just a single bolus dose of 13C tracer, which is immediately further diluted by the 12C pool upon injection and present only transiently, we found a small but significant increase in the labeled fraction in serine, ATP, ADP, and guanosine diphosphate (GDP) (and trends for higher labeling in AMP and guanosine monophosphate [GMP]) in the tumor tissue of IDO1-expressing versus EV controls (Figures 4B and S4C).