Variants in ARX cause a form of nonsyndromic X-linked intellectual disability, lissencephaly, Proud syndrome, infantile spasms without brain malformations, and Partington syndrome. X-linked lissencephaly-2 (LISX2) is characterized by structural brain anomalies, early-onset intractable seizures, severe psychomotor retardation, and ambiguous genitalia. Males are severely affected and often die within the first days or months of life, whereas females may be unaffected or have a milder phenotype. Proud syndrome is characterized by agenesis of the corpus callosum, severe intellectual disability, seizures, and spasticity. Males are severely affected, whereas females may be unaffected or have a milder phenotype. Partington syndrome is characterized by intellectual disability and variable movement disturbances. This laboratory performs Sanger sequencing for ARX.