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 resulting fragments were sequentially cloned into pBluescript SKII+ (Stratagene), cut with XmaI and NotI and ligated into pSB890 [15], yielding suicide plasmid pM1664.
Category: