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:As an example, we investigate the 27 nt pseudoknot PK1 of the upstream pseudoknot domain of the 3’-UTR of tobacco mild green mosaic virus, pseudobase ID PKB92 (Leathers et al., 1993).
Category: