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:Yeast strains with expression of a single hexose transporterencoding gene at a time allow scientists to study the properties of the individual permeases (Hamacher et al. 2002; Hahn-Hägerdal et al. 2006).
Category: