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:An increase in number of nAChRs at neuronal surfaces after prolonged nicotine exposure is probably mediated by several posttranslational mechanisms (Goriounova and Mansvelder, 2012a; Govind et al., 2012).
Category: