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:Butanol tolerance is a complex phenotype involving multiple loci (Makarova et al., 2006; Papoutsakis, 2008; Winkler et al., 2010), making the engineering of strains with enhanced tolerance to this solvent difficult.
Category: