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:Antigen receptor signaling during clonal expansion likely drives down Bcl-2 levels and hyper-induce Mcl-1 in responding CD8 T cells (Dunkle et al., 2011; Opferman et al., 2003).
Category: