Familial hemiplegic migraine (FHM) is an autosomal ominant classical migraine subtype that typically includes hemiparesis (weakness of half the body) during the aura phase. It can be accompanied by other symptoms, such as ataxia, coma and epileptic seizures. There is clinical overlap in some FHM patients with episodic ataxia type 2 and spinocerebellar ataxia type 6, benign familial infantile convulsions, and alternating hemiplegia of childhood. There are 3 known loci for FHM. FHM1, which accounts for approximately 50% of FHM patients, is caused by mutations in a gene coding for the P/Q-type calcium channel α subunit, CACNA1A. FHM1 is also associated with cerebellar degeneration. FHM2, which accounts for <25% of FHM cases, is caused by mutations in the Na+/K+-ATPase gene ATP1A2. FHM3 is a rare subtype of FHM and is caused by mutations in a sodium channel α-subunit coding gene, SCNA1. These three subtypes do not account for all cases of FHM, suggesting the existence of at least one other locus (FHM4). Many of the non-familial cases of hemiplegic migraine (sporadic hemiplegic migraine) are also caused by mutations at these loci.