The structure of the mod(mdg4) locus is identical in D. melanogaster and D. pseudoobscura. Twenty-one exons from the D. pseudoobscura gene are aligned side by side with the exons from the D. melanogaster gene, showing the same arrangement in both DNA strands. Brown boxes indicate the common mod(mdg4) exons, whereas the variable exons are represented in different colors. The arrows representing variable exons indicate their 5′to 3′ orientation with respect to the direction of transcription of the common exons. Red exons are in the same orientation as common exons, and exons present in the complementary DNA strand are shown in green. Black exons do not code for a zinc finger-like motif. Grey exons encode a BED finger domain. Exons represented by an empty arrowhead are described for the first time in this work. All exons are named after the mod(mdg4) mRNAs described in Buchner et al. (2000). Any other numbers in the D. melanogaster gene correspond to nucleotide positions in the sequence with accession nos. AE003734 and GI:7300718, -100 kb. Numbers in the D. pseudoobscura gene correspond to nucleotide positions in contig 4540.