Substantial exon rearrangements are necessary to explain structural differences between the mod(mdg4) gene in D. melanogaster and A. gambiae. Thirty-five variable exons from A. gambiae and 33 from D. melanogaster are shown by using the same color code as in Figure 1. Numbering of exons is as in Figure 1. Numbers in the A. gambiae mod(mdg4) gene correspond to nucleotide positions in the mosquito sequences with accession nos. AAAB01008851.1 and GI:19611880, -2000 kb. SD11801 and SD03001 are two cDNAs from the Drosophila Gene Collection 1 (Stapleton et al. 2002). Exons connected by V-shaped lines in the A. gambiae gene correspond to ESTs from the A. gambiae EST library. Lines connecting variable exons indicate orthology as deduced from Table 1.