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:This construction, pPUSIG, was mobilized by conjugation from Es. coli S17-1 λpir into Er. carotovora subsp. carotovora SCC3193 as described by de Lorenzo & Timmis (1994).
Category: