Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:1 is formed by UV or light irradiation of tryptophan [17] and, following activation of AhR, can increase expression of CYP1A1 to induce its own metabolism [7, 18] and provide a feedback mechanism to
Category: