Endometriosis (ET) tissues were enriched in ligands related to IDO1, Kyn, and AhR, as well as abundant expression of AhR on mast cells (MCs), which was hypothesized to act on MCs to release soluble factors to promote endometrial growth through the Trp-Kyn-AhR pathway, further leading to the development of ET (Mariuzzi et al., 2016). Here, IDO1 is linked to essential thrombocythemia.