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 (MIM 190198) (Sethi et al., 2010 [PubMed 19940119]).[supplied by OMIM, Mar 2010].