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:…a homologous exchange mutant of the Er. carotovora subsp. carotovora rpoS gene, the suicide vector pUT mini-Tn5 Cm was used. pUT mini-Tn5 Cm carries the λpir-dependent R6K ori, which does not replicate in bacteria that lack λpir, such as Er. carotovora subsp. carotovora (de Lorenzo & Timmis, 1994).
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