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 fact that the highly virulent Hypr strain is an old isolate, lacking most parts of the V 39NCR, have led to speculations that this virus has evolved by truncations and adapted to the laboratory environments during repeated cultivation in mammalian cells [30].
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