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:To illuminate the evolutionary history of GBP family, all newly identified genes combined with 13 previously described GBP genes (Olszewski et al. 2006; Degrandi et al. 2007) were used to reconstruct a phylogenetic tree.
Category: