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:VDR
downregulation was not found to be driven by copy number variation or mutation and
thus epigenetic mechanisms may be primarily responsible for altered expression(Long
and Campbell, 2015) (Long et al., 2014).
Category: