Identification of the Tryptophan Photoproduct 6-Formylindolo[3,2-b]carbazole, in Cell Culture Medium, as a Factor That Controls the Background Aryl Hydrocarbon Receptor Activity. - Öberg, Mattias; Bergander, Linda; Håkansson, Helen; Rannug, Ulf; Rannug, Agneta (Toxicological Sciences, 2005)