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:We obtained 658–667 bp of mtDNA control region sequences for each species from published data sets (Kulikova et al. 2005, 2012; Peters et al. 2005, 2008, 2012b, 2014; Lavretsky et al. 2014; Table S3, Supporting information), supplementing these data with new sequences.
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