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:Because a7 subunits are glycosylated (Gotti et al., 1992; Chen et al., 1998) and possess several phosphorylation consensus sites (Seguela et al., 1993), differences in oligosaccharide trimming or phosphorylation could cause the different a7 subunit pI values.
Category: