A degradation product of tryptophan upon exposure to visible light that activates the AhR (Kd = 70 pM) and is a substrate for CYP1A1, CYP1A2, and CYP1B1 (Kis = 15.4, 3.9, and 13.3 μM, respectively); used to study the immunomodulatory and host defense mechanisms of AhR signaling pathways