STXBP1 and neurodevelopmental disorder: STXBP1-related disorders are among the most common genetic epilepsies and neurodevelopmental disorders, with an estimated frequency of 1:30 000.1,2 The phenotypic landscape is characterized by neurodevelopmental delay in 95% and epilepsy in more than 80% of individuals.3 However, the wide range of neurodevelopmental features has made it difficult to delineate phenotypic subgroups,4–9 map genotype-phenotype associations, and understand treatment response.