STXBP1 and Intellectual disability: 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.