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:At these concentrations, protein expression of Cx-43 was reduced by 80% for up to 72 h (21), and protein expression of Tph1 was reduced by 90% for up to 72 h.
Category: