XYLT1 (xylosyltransferase 1) is a Golgi-localized glycosyltransferase that catalyzes the first step in chondroitin sulfate and dermatan sulfate proteoglycan biosynthesis by transferring D-xylose from UDP-D-xylose to serine residues on core proteins 1. The enzyme is essential for normal skeletal development, particularly long bone formation and chondrocyte maturation during ossification 2. XYLT1 deficiency causes Desbuquois dysplasia 2, characterized by impaired chondrogenic differentiation of mesenchymal stem cells, with disrupted extracellular matrix expression and premature chondrocyte hypertrophy 3. Beyond skeletal biology, XYLT1 participates in proteoglycan-mediated signaling: it post-translationally modifies biglycan to serve as a TLR3 ligand, activating interferon signaling in calcific aortic valve disease 4. In cancer biology, XYLT1 upregulation promotes early-stage lung adenocarcinoma metastasis by facilitating sulfated glycosaminoglycan conjugation of IκBα, enhancing NF-κB pathway activation 5. XYLT1 is transcriptionally regulated by AP-1 and Sp1 family transcription factors 6, and its expression is elevated during hepatic fibrosis where it promotes fibroblast proliferation and migration 7. These findings establish XYLT1 as a multifunctional regulator of skeletal homeostasis, inflammatory signaling, and pathological tissue remodeling.