XYLT1 (Xylosyltransferase 1): XYLT1 is an enzyme involved in the biosynthesis of proteoglycans, which are essential components of the extracellular matrix. It catalyzes the attachment of xylose to specific serine residues in proteoglycans, a crucial step in the formation of connective tissue. Mutations in XYLT1 can lead to skeletal and connective tissue disorders, highlighting its importance in tissue development and maintenance.