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:In addition, previous evidence indicated that higher expression of SIRT1 upregulated antiapoptotic proteins Bcl-2 and Bcl-xl, downregulated proapoptotic protein Bax, then increases cell viability in ischemia-reperfused myocardium [15].
Category: