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UVA1 phototherapy has been reported to mediate some of its effects via FICZ/AHR/MAPK signaling. A proof-of-concept study was performed to assess whether a topical solution of L tryptophan formulated to favor the generation 6-formylindolo[3,2-b] carbazole (FICZ) demonstrated activity at the aryl hydrocarbon receptor (AHR) as assessed by the intensity of cytochrome P450 1A2 staining. Ten participants applied a trial agent consisting of L- tryptophan, lecithin, polyvinyl alcohol and ethanol followed by 36 sessions of graduated sun exposure increasing to a total of 20 Joules per square centimetre. At the end of the trial, biopsies were taken from a treated site with photo-protected buttock skin used as a control. Assessment was via intensity of cytochrome P450 1A2 staining. An average baseline value of 53488923 units was obtained at the control site and 972214294 at the treatment site supporting the concept that this preparation displays activity at the AHR. As the components of this preparation are known to have UV absorbing properties and are potentially permissive of UVA-1 transmission, UV absorbance was assessed for natural sunlight and at 365 and 395nm confirming strong absorption in the UVB and UVA-2 spectra with partial transmission in the UVA-1 spectrum.