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:Promoter and coding regions of MDR pump genes having increased expression relative to parent strains were amplified by PCR and then sequenced in both directions using an automated method by the Applied Genomics and Technology Center, Wayne State University (Sanger et al., 1977).
Category: