Glycosyltransferase which elongates the O-linked glucose attached to EGF-like repeats in the extracellular domain of Notch proteins by catalyzing the addition of xylose. GXYLT1 is a xylosyltransferase (EC 2.4.2.-) that adds the first xylose to O-glucose-modified residues in the epidermal growth factor (EGF; MIM 131530) repeats of proteins such as NOTCH1. GXYLT1 (Glucoside Xylosyltransferase 1) is a Protein Coding gene. Diseases associated with GXYLT1 include Dowling-Degos Disease and Adams-Oliver Syndrome. Among its related pathways are Mucin type O-glycan biosynthesis. Gene Ontology (GO) annotations related to this gene include transferase activity, transferring glycosyl groups and UDP-xylosyltransferase activity. An important paralog of this gene is GXYLT2.