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 two plasmid-encoded MAR in C. necator JMP134 have been extensively studied (Kasberg et al., 1995; Muller et al., 1996; Seibert et al., 1993; Vollmer et al., 1993).
Category: