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:Prdm1 (encoding Blimp-1) has known roles in effector CD8+ and CD4+ T cell differentiation (Rutishauser et al., 2009; Shin et al., 2009; Johnston et al., 2009).
Category: