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 PCR product was digested with AgeI/NotI and ligated between the G and the L gene (similar to Roberts et al., 1999) into the vectors pVSVCT-1 and pVSVCT-9 (Schnell et al., 1998), which had been digested with XmaI/NotI.
Category: