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:…procedures for 2-dimensional gel electrophoresis (2DE) were based on those described previously (Cruz-Topete et al. 2011a; Cruz-Topete et al. 2011b; Cruz-Topete et al. 2011c; Christensen et al. 2011; Ding et al. 2011; Ding and Kopchick 2011; Okada et al. 2010; Sackmann-Sala et al. 2009; List et…
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