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 date, all characterized GBP genes are induced by IFNs and some of them also induced by lipopolysaccharide, interleukin-1b (IL-1b), or tumor-necrosis factor-a (Nguyen et al. 2002; Degrandi et al. 2007; Tripal et al. 2007).
Category: