Catalyzes the first step in the biosynthesis of chondroitin sulfate and dermatan sulfate proteoglycans, such as DCN. Transfers D-xylose from UDP-D-xylose to specific serine residues of the core protein (PubMed:15461586, PubMed:17189265, PubMed:24581741, PubMed:23982343). Required for normal embryonic and postnatal skeleton development, especially of the long bones (PubMed:24581741, PubMed:23982343). Required for normal maturation of chondrocytes during bone development, and normal onset of ossification (By similarity). L-seryl-[protein] + UDP-alpha-D-xylose = 3-O-(beta-D-xylosyl)-L-seryl-[protein] + H(+) + UDP Glycan metabolism; chondroitin sulfate biosynthesis. Glycan metabolism; heparan sulfate biosynthesis. Monomer. Detected predominantly in the Golgi apparatus. Widely expressed. Expressed at higher level in placenta, kidney and pancreas. Weakly expressed in skeletal muscle. Contains 7 disulfide bonds. N-glycosylated. The disease is caused by variants affecting the gene represented in this entry. The gene represented in this entry acts as a disease modifier. Activity is strongly reduced in seminal plasma of infertile men. Belongs to the glycosyltransferase 14 family. XylT subfamily.