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:The two-locus VRN-2/ VRN-1 epistatic interaction model of Yan et al. (2004a) provides a genetic mechanism to explain these opposite phenotypes and is supported by the results of this work and others (Danyluk et al., 2003; Murai et al., 2003; Trevaskis et al., 2003).
Category: