SCN1A and nervous system disorder: Compared with other monogenic childhood neurological diseases with reported cases exhibiting deleterious mosaic mutations, such as Rett syndrome (caused by mutations in MECP2)57–59, epilepsy in females with mental retardation (caused by mutations in PCDH19)39,40,60, mosaicism related to ATP1A337 or epilepsy-related neurodevelopmental disorders13, our DS cases with SCN1A mosaicism exhibit milder phenotypes compared with all the other probands who were detected with heterozygous mutations in SCN1A (Fig. 5).