FICZ (6-formylindolo(3,2b)carbazole) being a photooxidation product of tryptophan, might serve as a chemical messenger of light.
FICZ is a potent high affinity ligand of the aryl hydrocarbon receptor (AhR), which is a ligand-activated transcription factor that activates the expression of aromatic hydrocarbon metabolizing enzyme genes such as the CYP1A1 gene and has also been shown to have multiple additional roles in cell-cycle regulation, development and maturation of many tissues, control of inflammation, and immune response. FICZ is a photoproduct of tryptophan and is believed to be an endogenous AhR ligand. It is extremely potent, with a Kd value of 0.07 nM.