Consistent with these reports, we detected very low to non-detectable IDO2 and TDO2 proteins expressed in our CS and CR lung cancer cells as shown in Fig. 2A and Supplemental Figure S2A. However, IDO2 and TDO2 may be part of a compensatory/mutualistic mechanism induced when IDO1 is inhibited, providing another potential reason for the lack of efficacy presented in the phase 3 trial (ECHO-301) of epacadostat in combination with pembrolizumab for the treatment of melanoma [32]. Here, IDO2 is linked to lung cancer.