IDO1 and neoplasm: In mice receiving a control diet, IDO1 inhibition had no impact on tumor growth (Figure 4E), whereas in mice fed a diet lacking serine and glycine, IDO1 inhibition significantly slowed tumor growth (Figures 4F and S4G). We noted that the impact on tumor growth in vivo was not as substantial as the corresponding in vitro experiment (Figure 4C), possibly reflecting that serine levels can be more tightly controlled with an in vitro experiment.