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:However, there are examples of metabolic enzymes, such as glyceraldehyde-3-phosphate dehydrogenase and inosine monophosphate dehydrogenase (Zheng et al., 2003; Kozhevnikova et al., 2012), that perform double duty as transcriptional regulators.
Category: