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:FCWF cells and fMHV were obtained from Paul S. Masters (Albany, New York). fMHV is a recombinant MHV that contains the ectodomain of the S protein of feline infectious peritonitis virus, with the rest of the genes derived from MHV-A59 (Kuo et al, 2000).
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