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:F proteins are acid pH dependent and contain a predicted signal peptide at the amino terminus, a transmembrane domain near the carboxyl terminus, up to 11 conserved cysteines, and a furin cleavage site (40, 51, 56).
Category: