XXYLT1 (xyloside xylosyltransferase 1) is an endoplasmic reticulum-localized, type II membrane-bound glycosyltransferase that catalyzes alpha-1,3-xylosylation of O-linked glucose-glycans attached to epidermal growth factor (EGF)-like repeats 1. The enzyme adds a second xylose residue to xylose-α1,3-glucose disaccharides, generating the terminal trisaccharide modification xylose-xylose-glucose 1. Primary substrates include Notch proteins and coagulation factors such as factor IX 1. Structurally, XXYLT1 employs an SNi-like retaining mechanism, with its active site inducing significant conformational changes in EGF-repeat acceptor substrates to ensure specificity 2. In mammalian systems, XXYLT1-mediated xylosylation negatively regulates NOTCH1 activation; genetic deletion of XXYLT1 enhanced NOTCH1 signaling and reduced cell proliferation, suggesting the enzyme fine-tunes Notch activity 3. Beyond Notch modification, a long noncoding RNA antisense to XXYLT1 (XXYLT1-AS2) regulates endothelial cell function by targeting the RNA-binding protein FUS, with implications for atherosclerosis pathogenesis 4. Clinically, XXYLT1 variants are associated with hemostatic traits and cardiovascular events 5, while aberrant XXYLT1 methylation is linked to lung adenocarcinoma and endometrial stromal tumors 67.