The ATP1A3-related disorders are clinically heterogeneous and include a spectrum of at least 3 distinct, although overlapping, phenotypes: rapid-onset dystonia-parkinsonism (RDP)2; alternating hemiplegia of childhood (AHC)1; and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS).3 Although rare, these conditions are important because they are generally severe, including paroxysmal events and chronic severely disabling neurologic deficits,4 with an increased rate of premature mortality. The gene discussed is ATP1A3; the disease is cerebellar ataxia.