Surprisingly, although Ido1, Ido2 and Tdo2 were barely expressed in MEPs (Fig. 4a and Extended Data Fig. 4a), a high level of Kyn was found in the MEPs of tumor-bearing mice and a very low Kyn level was present in the MEPs of tumor-free mice (Fig. 4b and Extended Data Fig. 4b), suggesting that MEPs from tumor-bearing hosts might use exogenous Kyn to activate AhR. Here, TDO2 is linked to neoplasm.