ATP1A3 and alternating hemiplegia of childhood: For individual U018, a novel pathogenic variant was identified in ATP1A3. This gene has been known and thought to be the cause of three distinct diseases since 2004: rapid-onset dystonia Parkinsonism (RDP), alternating hemiplegia of childhood (AHC), and CAPOS (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss) syndrome31–34.