As noted earlier, to our knowledge, to date 6 EA phenotypes have been described, but only EA1 and EA2 have been reported in large numbers of families (Table). Mutations in the potassium channel gene KCNA1 lead to the EA1 phenotype, which is characterized by brief episodes of ataxia with interictal myokymia,7 while EA2, caused by mutations in the calcium channel gene CACNA1A, is characterized by more prolonged episodes of ataxia with interictal nystagmus.8 The EA3 phenotype was described in a single large Canadian family with episodic vertigo, tinnitus, and ataxia and was recently found linked to chromosome 1q42.9 The EA4 phenotype, also called periodic vestibulocerebellar ataxia, was described in 2 North Carolina kindreds with late-onset vertigo and ataxia, as well as interictal nystagmus. Linkage analysis ruled out the EA1 and EA2 loci, but so far no genome-wide scan has been reported.10 The EA5 phenotype was identified when a series of families with EA were screened for mutations in the calcium channel β4-subunit, CACNB4, on chromosome 2q.11 This family had clinical features similar to those of EA2, but mutations in CACNA1A were ruled out. Complicating matters, the same mutation was found in a German family with generalized epilepsy (but no ataxia), and functional studies showed only subtle changes in calcium channel function. Finally, EA6 was described in a single child with EA, episodes of hemiplegia, and seizures in which a rare mutation was identified from a screen of the candidate gene, SLC1A3, a glutamate transporter localized to astrocytes.12 The mutation was de novo, and functional studies of the mutated protein showed an almost complete loss of function.