Glycosyltransferase required for the biosynthesis of the tetrasaccharide (GlcA-Gal-Gal-Xyl-)Ser core linker of heparan sulfate and chondroitin sulfate. Required for embryonic development (PubMed:11005858). Involved in vulval epithelium invagination (PubMed:9927677). Required for axon regeneration after injury (PubMed:31371405). 3-O-(beta-D-xylosyl)-L-seryl-[protein] + UDP-alpha-D-galactose = 3-O-(beta-D-galactosyl-(1->4)-beta-D-xylosyl)-L-seryl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Belongs to the glycosyltransferase 7 family.