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:Previous research, as well as our study results, agree with the hypothesis that the expression of solanaceous PT4 genes are specially induced by AM (Chen et al. 2007; Drissner et al. 2007; Karandashov and Bucher 2005; Nagy et al. 2005; Poulsen et al. 2005).
Category: