IDO activity, on the other hand, enhances Trp uptake into tumours by inducing the Trp transporter responsible for 50% of Trp uptake [98] and, whereas these latter authors [98] suggested that the stimulus for expression of this transporter is the Trp depletion induced by IDO, evidence exists for up-regulation of SLC7A5 by the AhR [137–139] secondarily to IDO induction, thereby representing a positive feed-forward mechanism for AhR activity [140], as illustrated in the proposed IDO/TDO-AhR-SLC7A5 axis in Figure 5 here. This evidence concerns the gene IDO1 and neoplasm.