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:…for ovarian and other similar cancer cell lines have shown an increase in infectivity through CAR-independent transduction, achieving higher reporter gene expression by several orders of magnitude in the primary tumor cells (Dmitriev et al., 1998; Kasono et al., 1999; Vanderkwaak et al., 1999).
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