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 building glycosaminoglycan chains, which support cell signaling, adhesion, and movement within the extracellular matrix. Changes in XXYLT1 can affect these processes and have been linked to developmental disorders.