Intriguingly, symptoms in these four human disorders are episodic, and believed or known to be precipitated by external or internal stimuli: the paroxysmal muscular disorders, paramyotonia congenita/myotonia congenita and myotonia fluctuans (SCN4A) (McClatchey et al. 1992; Ricker et al. 1994); near-sudden infant death syndrome (SIDS) caused by the cardiac condition long QT syndrome type 3 (LQT3) (SCN5A) (Schwartz et al. 2000) and severe myoclonic epilepsy of infancy (SMEI) (SCN1A) (Claes et al. 2001; Escayg et al. 2000). Here, SCN5A is linked to long QT syndrome 3.