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:Mcl-1 deletion invariably results in apoptosis of naïve T cells (Opferman et al., 2003), which precludes experiments to assess the cell survival-independent roles of Mcl-1 in T cells.
Category: