FIG. 2. The AVP and OT genes are localized on human chromosome 20 and are separated by only 8–12 kb of intervening DNA sequence. This DNA domain undoubtedly results from the genomic duplication of a progenitor gene (i.e., the vasotocin gene), an event estimated to have occurred 350–450 million years ago. The transcriptional orientations of the two genes are inverted (depicted by the arrows in the lower diagram) such that the promoter region for each gene lies within the flanking regions of the duplication domain. The more detailed schematic of the AVP and OT genes, shown in the upper portions of the figure, depicts the highly conserved exon–intron structures. Each gene is comprised of three exons (lettered boxes) separated by two intervening sequences (intron 2 is not labeled). Exon B of the AVP gene is the location for several mutations found to cause inherited forms of autosomal dominant hypothalamic diabetes insipidus.