UniProt Summary: Beta-1,3-N-acetylglucosaminyltransferase that plays a role in the elongation of specific branch structures of multiantennary N-glycans. Has strong activity towards tetraantennary N-glycans and 2,6 triantennary glycans. {ECO:0000269|PubMed:15620693, ECO:0000269|PubMed:15917431}.