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
isoforms differ in sequence primarily by the inclusion of an extended N-terminus (‘short
consensus repeats’ or ‘sushi domains’) on the GABAB(1a) isoform (Kaupmann et al. 1998a).
Category: