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:treatment with Fgf8b is shown, because Fgf8b, the most potent isoform of Fgf8, is the only gene of the studied FGF family members that has been found to be crucial in mesoderm development in the mouse (Guo and Li, 2007).
Category: