We may have identified the link between these observations as our results reveal the existence of an IDO-AHR-IL-6-STAT3 autoactivation loop in human cancer cells, which may underlie the induction of IL-6 by IDO in mouse tumors (via the AHR), the induction of IDO by IL-6 in rat hippocampus (via phosphorylated and acetylated STAT3) and the requirement of the AHR for IDO expression in mouse DC. The gene discussed is IDO1; the disease is cancer.