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:The mouse neuronal hybrid cell line MN-1 was cultured and transfected as previously described (Drews et al. 2005), using 50 ng of test plasmid, 10 ng of control plasmid (pRLSV40, Promega), and the Fugene 6 transfection reagent (Roche Molecular Biochemicals, Indianapolis, IN).
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