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 4.3, 6.36, and 9.25 (ectodomain) and alternative exon 17.1 (transmembrane domain).