STXBP1 and Down syndrome: STXBP1 syndrome (syntaxin-binding protein 1 syndrome) is a severe infantile epileptic encephalopathy (EIEE4, OMIM: #612164) [1] caused by mutations in the STXBP1 gene, which encodes the syntaxin-binding protein 1, also known as Munc18-1 (UniProt: P61764) [2,3,4,5].