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:At 6 days after LCMV infection, virus-specific Tfh cells (CXCR5+ SLAMlo) and Th1 cells (CXCR5 SLAMhi) (Choi et al., 2013; Johnston et al., 2009) were isolated by flow cytometry and deep sequenced for differential representation of the shRNAs.
Category: