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:U-box domain does not depend on zinc binding to keep its stability, but have evolved instead networks of hydrogen bonds and salt bridges formed by the corresponding residues at metal ligand positions (Hatakeyama et al., 2001; Nordquist et al., 2010; Wiborg et al., 2008).
Category: