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 CB served as positive control tissue for mGluR1, mGluR2, and mGluR4 (15, 30), whereas the OB prominently expresses mGluR6, mGluR7, and mGluR8 (5, 23, 27).
Category: