DS patients with the same pathogenic variant, even within the same family, can show different clinical phenotypes, ranging from seizure-free to severe forms of DS.2,12 Studies investigating correlations between SCN1A variants and phenotype severity have not found a reliable correspondence.13, –15 Another unanswered question concerns mechanisms underlying the developmental delay in language, motor function, learning, and social skills that heavily affect many DS patients. Here, SCN1A is linked to Dravet syndrome.