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:Chinese hamster ovary (CHO) cells were stably transfected with either the human TNF-a gene cloned into a mammalian expression vector (TNF-a cells) or the mammalian expression vector alone (CHO cells, control) as described previously (5, 7, 8).
Category: