A 4.5kb Dscam1 promoter fragment drives expression of a cDNA (exons 3-17) - genomic DNA (exons 18-24) hybrid sequence encoding a Dscam1 isoform. The sequence contains alternative exons 4.2, 6.18, and 9.22 (ectodomain) and alternative exon 17.2 (transmembrane domain).