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:Transient transfection of HeLa cells using Lipofectamine 2000 (Invitrogen) and infection of primary HFFs with HCMV laboratory strain AD169 or clinical isolates R3 (Iso 3) and R4 (Iso 4) was performed as described elsewhere (Marschall et al., 2000; Efferth et al., 2002; Herget et al., 2004; Schregel et al. 2007).
Category: