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 contrast to most plant PPR proteins identified so far, which contain 12 PPR repeats on average (Lurin et al., 2004; Tourasse et al., 2013), PPR7 consists of only four PPR units, followed by a very short, positively charged conserved tail (Supplemental Figure 1).
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