In 1997, Miura et al. isolated a novel homeobox gene in zebrafish and mice with striking homology to the Drosophila gene aristaless. They called this gene Arx (aristaless-related homeobox gene) and showed that it was highly conserved between zebrafish and mice and expressed in the developing central nervous system. The human orthologue of this gene, ARX, is located on the short arm of the X chromosome (Xp22.13). It consists of five exons and encodes a 562 amino acid protein that may be a nuclear transcriptional regulator. It is expressed in the fetal and adult brain, with much higher expression in the fetal brain. In the fetal brain the ARX gene is primarily expressed in neuronal precursors in the germinal matrix and ventricular zone at all stages of development but also in the subventricular zone, cortical plate, caudate nucleus, putamen, substantia nigra, cingulate, and hippocampus. It is also expressed in fetal testes and adult heart, skeletal muscle, and liver.