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:Crystal structures of some theta GSTs have indicated that the GSH-binding site may be sited further inside the protein than in other classes of GST, and that conventional GSH-aYnity matrices may be unable to bind the active site (Hayes and Pulford, 1995).
Category: