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:As stated earlier, GABAB(1a) and GABAB(1b) are the most abundantly expressed variants of the GABAB(1) subunit (Kaupmann et al. 1997), both of which form functional heterodimers with the GABAB(2) subunit (Benke et al. 1999; Kaupmann et al. 1998a).
Category: