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:The GABAB(1) subunit is predominantly expressed in the CNS as one of two isoforms:
GABAB(1a) or GABAB(1b) (Kaupmann et al. 1998a).
Category: