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:Overexpression of Skp1 or absence of pathway activity drives the O2 requirement up to 1821% (near ambient level), whereas decreased Skp1 or overexpression of PhyA drives the O2 requirement down to 5% or less [5,10,11].
Category: