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:Isolation of monoclonal antibodies (Mabs) from HIV-infected individuals, has identified four main epitope targets—the membrane proximal external region (MPER) in gp41, targeted by Mabs such as 10E8 [13], 4E10 [14], and 2F5 [15], the CD4-binding site, targeted by Mabs such as VRC01 [16], NIH45-46W [17,18], and HJ16 [19], glycopeptide residues in the V1/V2 region and also the V3 loop, targeted by Mabs such as PG9 [20] and PGT128 [21], respectively.
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