A previously unknown mammalian UDP-N-acetylglucosamine:.alpha.-6-D-mannoside .beta.-1,6-N-acetylglucosaminyl-transferase (termed GlcNAc T-Vb herein) coding sequence, protein, recombinant host cells and antibodies which specifically bind GlcNAc T-Vb are described. In particular, GlcNAc T-Vb of mouse is disclosed.