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ARXis a paired-type homeobox gene located on the X chromosome that contains five exons with four polyalanine (PolyA) tracts, a homeodomain, and a conserved C-terminal aristaless domain. Studies in humans have demonstrated remarkable pleiotropy: malformation phenotypes are associated with protein truncation mutations and missense mutations in the homeobox; nonmalformation phenotypes, including X-linked infantile spasms (ISS), are associated with missense mutations outside of the homeobox and expansion of the PolyA tracts.