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:Two PTF1 binding sites are present in the 2.3 kb Ptf1a regulatory sequence Sequence analysis of the two highly conserved peaks in the 2.3 kb enhancer revealed the presence of two PTF1 binding sites, referred to here as the proximal (P) and the distal (D) PTF1 sites (Masui et al., 2008).
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