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:In rare cases where PCR did not yield amplicons, the primers 63f (50 CAG GCC TAA CAC ATG CAA GTC 30) and 1387r (50 GGG CGG WGT GTA CAA GGC 30) (Marchesi et al. 1998) were used.
Category: