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:Model I-2 (n = 4) and -5 (n = 4) were directly injected with 50 mg of the human AE1-targeted siRNAexpressing vector and 50 mg of the scramble vector into the tail vein, respectively, once every 3 days for a total of seven doses, as described previously (Garg et al., 2008).
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