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:In addition, there may be differences in receptor expression patterns between different
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strains of mice and between mice and rats (Hajos and Freund, 2002; Hoffman et al., 2005; Takahashi and Castillo, 2006).
Category: