XYLT1 (Xylosyltransferase 1): XYLT1 is an enzyme involved in the biosynthesis of proteoglycans, essential components of the extracellular matrix. It catalyzes the addition of xylose to specific serine residues in proteoglycans, a key step in connective tissue formation. Mutations in XYLT1 can cause skeletal and connective tissue disorders, underscoring its role in tissue development and maintenance.