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:It is noteworthy that accumulation of ethidium bromide may be affected by causes other than diminished efflux pump activity; as an example, increased permeability to ethidium bromide due to physical changes of the cell envelope.(7,8,25) If one is to assess efflux of ethidium bromide and conditions that affect it, one must first employ conditions which promote the accumulation of ethidium bromide.
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