Catalytic component of CHSY1-CHPF2 and CHSY1-CHPF chondroitin sulfate synthase complexes (PubMed:41298522). Has both beta-1,3- glucuronic acid and beta-1,4-N-acetylgalactosamine transferase activity. Transfers glucuronic acid (GlcUA) from UDP-GlcUA and N- acetylgalactosamine (GalNAc) from UDP-GalNAc to the non-reducing end of the elongating chondroitin polymer (PubMed:11514575, PubMed:12716890). Involved in the negative control of osteogenesis likely through the modulation of NOTCH signaling. {ECO:0000269|PubMed:11514575, ECO:0000269|PubMed:12716890, ECO:0000269|PubMed:21129727, ECO:0000269|PubMed:41298522}. This is the function of CHSY1 (chondroitin sulfate synthase 1, ENSG00000131873).