STXBP1 and developmental and epileptic encephalopathy, 4: Pathogenic variants in STXBP1, a gene involved in the exocytosis of synaptic vesicles, cause early infantile epileptic encephalopathy type 4 (EIEE4), or infantile spasms (IS) with non-syndromic encephalopathy, and frequent co-occurrence of MD [26].