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:For example, the contributions of the EGF and HRG bound HER1HER3 dimer to the EGFR and HER3 phosphorylation signals at the cell surface are respectively expressed as [R13EHs]*pf13ehs and [R13EHs]*pf31ehs where [R13EHs] is the dimer abundance and pf13ehs and pf31ehs are cell surface phosphorylation factors.
Category: