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:CSE [prepared by dissolving the collected smoke particulates in dimethyl sulfoxide (DMSO)] or cytokine mix (TNF- and IFN- ) induced MMP-12 mRNA in HBEC (177, 178).
Category: