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 used two luxAB reporter strains, strain FAM431, which contains the fulllength glnN promoter fused to luxAB, and FAM84, which only retains the NtcA-binding site in its promoter fused to luxAB (Aldehni & Forchhammer, 2006).
Category: