With increased clinical genetic testing, an increasing number of individuals are recognized to have developmental delay and intellectual disability without epilepsy within STXBP1-related disorders.1,5,8 As initial cohort studies tend to include more severely affected individuals, interpretation of outcomes can be variable when considering selection bias.1,17,40 In contrast, 30% of our cohort did not have seizures, pointing to the need to delineate developmental end points as an additional outcome measure complementary to epilepsy end points. This evidence concerns the gene STXBP1 and epilepsy.