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:Cells at passages 3–5 were labeled with FITC conjugated anti-rat CD44, anti-rat CD45, anti-rat CD90 antibodies (Caltag Laboratories Inc, USA) and characterized by fluorescence activated cell sorting (FACSCalibur, Becton-Dickinson, USA) [20].
Category: