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:Accumulating evidence suggests that cytokines including IL-1a/b, IL-2, IL-5, IL-6, IL-8, IL-10, TNF-a, TNF-b, and IFN-c are secreted in the urine following instillation ofBCG [2, 3, 4, 6, 7, 9, 11, 12, 14, 15, 23, 26].
Category: