6-Formylindolo[3,2-b]carbazole (FICZ) is a chemical compound with the molecular formula C19H12N2O. It is a nitrogen heterocycle, having an extremely high affinity (Kd = 7 x 10−11M) for binding to the aryl hydrocarbon receptor (AHR). It was originally identified as a photooxidized derivative of the amino acid tryptophan and suggested to be the endogenous ligand of the AHR. Later, also enzymatic formation of FICZ was shown.