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:Inasmuch as previous studies have shown that COX-2 is induced in response to hypoxia in HUVEC in vitro, these cells were used as a positive control in the hypoxia experiments (19, 40).
Category: