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, a crucial step in forming connective tissues. Mutations in XYLT1 can lead to skeletal and connective tissue disorders, highlighting its vital role in tissue development and maintenance.