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:Although international guidelines are not available on the topic, in our routine experience sampling of different tumor areas by core biopsy (three biopsies on average) is helpful to define the precise nature of the lesion, the different tumor histological types present in the mass, and the heterogeneity in the expression of predictive markers (Marchio and Sapino, 2011).
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