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:The effect of O2 appears to be mediated at least in part by prolyl 4-hydroxylation of Skp1, because elevated O2 levels are required by phyA – and Skp1-overexpression strains, and lower O2 is required by PhyA overexpression and Skp1B cells [10,24].
Category: