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:…for ptDNA may be complicated by the presence of NUPTs (nuclear sequences of plastid origin) (Roark et al. 2010; Kumar and Bendich 2011; Zheng et al. 2011), and the same difficulty may be faced in measuring mtDNA due to NUMTs (nuclear sequences of mitochondrial origin) (Timmis et al.…
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