STXBP1 and early-infantile DEE: Disease-causing alterations in STXBP1 are among the most common causes of neurodevelopmental disorders and epilepsy with an estimated frequency of at least 1:30 000.1,2 The role of STXBP1 variants in human neurological disorders was initially identified in Ohtahara syndrome,3 but subsequent studies have identified a broader range of other neurological features.2,4,5 However, in contrast to other genetic causes of neurodevelopmental disorders,6,7STXBP1-related disorders have received relatively little attention to understanding genotype–phenotype correlations.