Mutations in the voltage-gated sodium channel gene SCN1A are a major cause of severe myoclonic epilepsy of infancy (Dravet syndrome) and generalized epilepsy with febrile seizures plus. This study reports the identification of six de novo SCN1A mutations in patients with severe myoclonic epilepsy of infancy, including a tetranucleotide deletion in exon 26. The same deletion was previously observed in two unrelated patients and appears to result from slipped-strand mispairing of a direct repeat during deoxyribonucleic acid replication. Review of the literature indicates that recurrent mutations account for 25% of SCN1A mutations in severe myoclonic epilepsy of infancy, including six sites of deamination at CpG dinucleotides.