(A) Dscam-hv genomic DNA for Drosophila melanogaster. The gene consists of 20 constant exons (shown as black lines), mutually exclusive alternative splicing occurs for exons 4 (red lines), 6 (blue lines), 9 (green lines) and 17 (purple lines); one of 12 exon 4 alternatives, one of 48 exon 6 alternatives, one of 33 exon 9 alternatives and one of two exon 17 alternatives are present in each mRNA. This enables the vast number of 12 × 48 × 33 × 2 = 38,016 potential splice variants. (B) Dscam-hv mRNA. Constant exons are shown as white boxes. Exons that undergo mutually exclusive alternative splicing follow the same colour scheme as for the genomic structure. Endodomain exons 19 and 23 can be contained or lacking , which increases the number of potential isoforms to 4 × 38,016 = 152,064. (C) Dscam-hv protein structure for D. melanogaster. The alternatively spliced exons encode the N-terminal half of Ig2 (exon 4 in Drosophila); the N-terminal half of Ig3 (exon 6 in Drosophila), all of Ig7 (exon 9 in Drosophila), and the transmembrane domain (Exon 17 in Drosophila (figure after ).