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. Protein modification; protein glycosylation. Interacts with B3GNT2; this interaction greatly increases B3GNT2 catalytic activity, independently of B3GNT8 enzymatic activity. Highly expressed in small intestine, pancreas, spleen, bone marrow, lung, throat, and ileum, and weakly in fetal brain, cerebellum, heart, liver, tongue, breast, uteri, and testis. Not detected in colon. Differentially expressed in human tumor cell lines. Up-regulated in colon cancer. Belongs to the glycosyltransferase 31 family.