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:In recombinant systems, many studies attest to a lack of pharmacological differences
between the GABAB(1a,2) and GABAB(1b,2) isoforms (Kaupmann et al., 1998a; Malitschek et al., 1998; Brauner-Osborne and Krogsgaard-Larsen, 1999; Green et al., 2000), although there
are also a few findings to the…
Category: