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:Second, we have previously shown that STAT3 expression was not changed by modulating RKIP expression in 293T cells [60] and third, because 293T cells have a higher transfection efficiency with the plasmids we were using, especially under the serum free conditions used in the experiments described below.
Category: