ATP1A3 and alternating hemiplegia of childhood: Mutations in the α3 catalytic subunit of Na+/K+ adenosine triphosphatases, encoded by ATP1A3, are classically associated with three distinct neurological syndromes: rapid‐onset dystonia parkinsonism (RDP); alternating hemiplegia of childhood (AHC); and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS).