XXYLT1 (Xyloside Xylosyltransferase 1) is a gene that encodes an enzyme responsible for modifying proteins by adding xylose sugars to proteoglycans. This modification is essential for constructing glycosaminoglycan chains, which support cell signalling, adhesion, and movement within the extracellular matrix. Alterations in XXYLT1 can impact these processes and have been associated with developmental disorders.