Severe Myoclonic Epilepsy in Infancy (SMEI) is ail intractable epileptic syndrome with onset in the first year of life and is commonly caused by de novo mutations in the SCN1A gene, encoding the alpha 1-subunit of the neuronal voltage-gated sodium channel. We report two unrelated families in which probands were affected by SMEI and their parents showed a single febrile seizure during early childhood or no neurological symptoms. semiquantitative analysis of SCN1A mutations allowed the detection of a somatic and germline mosaicism in one of the parents. The Study provides the first example of parental mosaicisms in SMEI and opens a new insight into the phenotypic variability and complex inheritance of this condition. The identification of germline mosaicisms has important consequences in genetic Counseling of SMEI when SCN1A mutations appear to Occur de novo with standard screening methods. (c) 2006 Elsevier Inc. All rights reserved.