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:‚Ä¶endosperm harvested 18 d after pollination and stored at 808C were cut into small pieces and treated with 2% (w/v) formaldehyde in ice-cold PBS buffer for 4 h. Cross-linking was quenched by addition of 2 M glycine for 1 h. Total proteins were then extracted as described by Boehlein et al. (2005).
Category: