Both DYT12 and AHC mutations lead to reduced ATPase activity, whereas AHC mutations did not affect the protein expression level that was observed for DYT12. Recently, a third allelic disorder for ATP1A3 was identified, episodic Cerebellar Ataxia, Areflexia, Optic Atrophy, and Sensorineural Hearing Loss (CAOS) by exome sequencing (45). Here, ATP1A3 is linked to hereditary optic atrophy.