PURPOSE:ARX, the aristaless-related homeobox gene, is implicated in cerebral, testicular, and pancreatic development. ARX mutations are associated with various forms of epilepsy, developmental delay, and ambiguous genitalia in humans. A mouse model that recapitulates X-linked lissencephaly with ambiguous genitalia (XLAG) is far from elucidating the substrate for phenotypes that different ARX mutations cause. Moreover, despite phenotypic pleomorphism associated with X-linked dominant ARX mutations, heterozygous female carriers have not been thoroughly studied. Reviewing records of patients with ARX mutations, infantile epilepsies, and psychomotor retardation, we analyzed a family harboring a novel ARX mutation with different phenotypes in males and females, including Ohtahara syndrome. METHODS:Children's Hospital Boston patient records were retrospectively screened for patients with infantile epileptic encephalopathies who underwent ARX sequencing based on clinical suspicion. Identified families were analyzed for genetic and neuropsychiatric phenomena. KEY FINDINGS:The proband was a male with Ohtahara syndrome, ambiguous genitalia, psychomotor delay, and central nervous system dysgenesis due to a novel ARX mutation in exon 5, causing a frameshift in the aristaless domain. Heterozygous females demonstrated neurocognitive/psychiatric phenomena including learning difficulties, anxiety, depression, and schizophrenia. SIGNIFICANCE:This is the first reported case of Ohtahara syndrome with abnormal genital and psychomotor development in the setting of this novel ARX mutation in exon 5. Based on the unique phenotype of the proband and on the presence of heterozygous females with neurocognitive/psychiatric ailments, this study describes the potential roles for ARX mutations in epilepsy and neuropsychiatric disease, underscoring the importance of ARX in interneuron development, cerebral electrical activity, cognition, and behavior.