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:MtrF and NGO1024 appear to be linked in a regulatory sense, in that the transcripts for these proteins are controlled directly or indirectly by at least four regulatory elements, including NrrF, Fur, and two additional transcriptional repressors, MtrR and MpeR (57, 58).
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