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:Although MB morphogenesis involves Dscam variants lacking exons 19 and 23, past studies have shown that the Dscam 19 23 transgene, no matter which ectodomain it carries and whether it is fused with GFP at the C terminus or not, can effectively prevent sister branches from extending into the same axon bundle in single-cell clones of Dscam mutant neurons (Wang et al., 2004; Zhan et al., 2004; Soba et al., 2007).
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