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:Enterococcus faecium also maintains copper homeostasis through membrane transporter proteins of the P-type ATPase family, which are encoded by the chromosomal operon copYZAB which is similar to copYZAB described in Enterococcus hirae (Solioz and Vulpe 1996; Wunderli-Ye and Solioz 1999).
Category: