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:…studies that the GABAB(1a) isoform was predominantly expressed in the granule cell layer, while the GABAB(1b) isoform was expressed mainly in the Perkinje cells (Billinton et al. 1999; Bischoff et al. 1999; Fritschy et al. 1999; Kaupmann et al. 1998b; Liang et al. 2000;
Poorkhalkali et al. 2000).
Category: