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:Gene expression For quantification of mRNA expression in murine samples, cells/tissues were homogenized in TRIzol (Invitrogen), and RNA was extracted and cDNA was generated and quantified as previously described (91) using a Light Cycler 480 II (Roche).
Category: