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:Mammalian T1Rs and T2Rs, which are G-protein coupled receptors (GPCRs), are expressed in taste bud cells (Adler et al. 2000; Hoon et al. 1999).
Category: