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:Hepatocytes were isolated from adult male Sprague-Dawley rats and adult male mice C57/BL6 wild type; C57/BL6-lpr (FAS receptor null); C57/BL6-gld (FAS ligand mutant); by the two-step collagenase perfusion technique (Kamath et al., 1999; Park et al., 2000a,b).
Category: