SCN1A and Global developmental delay: 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.