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:Increasing the number of EPIYA motifs leads to an increase in the level of CagA phosphorylation (Higashi et al., 2002a; Argent et al., 2004, 2008a; Zhang et al., 2005; Naito et al., 2006).
Category: