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:Previous studies have shown that sialome genes were subject to strong ancient selective pressures that have resulted in species-specific adaptations (Varki 2001; Angata et al. 2004; Altheide et al. 2006; Varki & Gagneux 2009), suggesting that they likely evolved in response to ancient pathogens The function of sialome genes, i.
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