Glycosyltransferase that catalyzes the transfer of an N-acetylglucosamine (GlcNAc) moiety in beta1-6 linkage from UDP-GlcNAc onto mucin-type core 1 O-glycan to form the branched mucin-type core 2 O-glycan. The catalysis is metal ion-independent and occurs with inversion of the anomeric configuration of sugar donor (PubMed:9881978, PubMed:12954635, PubMed:22056345). Selectively involved in synthesis of mucin-type core 2 O-glycans that serve as scaffolds for the display of selectin ligand sialyl Lewis X epitope by myeloid cells, with an impact on homeostasis and recruitment to inflammatory sites (PubMed:9881978). Can also act on glycolipid substrates. Transfers GlcNAc moiety to GalGb4Cer globosides in a reaction step to the synthesis of stage-specific embryonic antigen 1 (SSEA-1) determinant (PubMed:7983056). Can use Galbeta1-3GalNAcalpha1-R and Galbeta1-3GalNAcbeta1-R oligosaccharide derivatives as acceptor substrates (PubMed:7983056). O(3)-[beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl]-L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-{beta-D-galactosyl-(1->3)-[N-acetyl-beta-D-glucosaminyl-(1->6)]-N-acetyl-alpha-D-galactosaminyl}-L-seryl-[protein] + H(+) + UDP O(3)-[beta-D-galactosyl-(1->3)-N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-{beta-D-galactosyl-(1->3)-[N-acetyl-beta-D-glucosaminyl-(1->6)]-N-acetyl-alpha-D-galactosaminyl}-L-threonyl-[protein] + H(+) + UDP globoside GalGb4Cer + UDP-N-acetyl-alpha-D-glucosamine = globoside GlcNAc-(beta1->6)-GalGb4Cer + H(+) + UDP ganglioside GA1 + UDP-N-acetyl-alpha-D-glucosamine = ganglioside beta-DGlcNAc-(1->6)-GA1 + H(+) + UDP Inactivated by thiol-reactive agents. Inhibited by free UDP. Protein modification; protein glycosylation. Glycolipid biosynthesis. Interacts with GOLPH3; may control GCNT1 retention in the Golgi. Also detected in the trans-Golgi network. Expressed in kidney, liver, stomach, spleen, lung and brain. N-glycosylated. Deficient mice show normal organogenesis and fertility. The loss of core 2 O-glycans on leukocytes is associated with leukocytosis and impaired neutrophil recruitment at the inflammatory site, while lymphocyte homing to peripheral lymphoid organs is not affected. Belongs to the glycosyltransferase 14 family. core 2 beta 6 GlcNAc T1