The Exon–Intron Structure of HumanLHX1 Gene

We have determined the genomic structure of the humanLHX1 gene, a member of the LIM/homeobox (Lhx) gene family. The transcript is assembled from five exons, which are separated by introns ranging in size from 93 nt to 2.3 kb. The two LIM domains are entirely contained in the first and second exons, respectively, while the homeodomain is split into exons three and four. This structure closely parallels the organization of other mouse and human Lhx genes whose genomic structure is known. An exception is the mouse and humanisl1 genes, whose homeodomain does not contain introns. An intron at the same position also occurs in theXlim1 gene as well as in other homeobox genes, such asevx1 andevx2, suggesting that this intron insertion represents an ancestral event, from which homeobox genes of different families originated. In this context, evolution of theLhxgene family probably involved the shuffling of this intron-containing homeobox in the proximity of a LIM-only gene, whileIsletgenes were formed either by the shuffling of an intronless homeobox to the same LIM domain or, alternatively, by intron loss during their evolution.