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:‚Ä¶out in PBS containing 50 mg DEAE-dextran ml21 and 2 % FCS. Inoculations with a low dose (m.o.i. ¬° 0.05) of MERS-CoV or SARS-CoV (strain HKU-39849; Zeng et al., 2003) were carried out directly in EMEM containing 2 % FCS. Virus titrations by plaque assay were performed as described previously‚Ä¶
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