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:Several lines of evidence suggest that REIC down-regulation in cancers is caused by promoter methylation (Ding et al., 2009; Yu et al., 2009; Veeck et al., 2009; Yang et al., 2010).
Category: