Branching enzyme that converts linear into branched poly-N-acetyllactosaminoglycans. Introduces the blood group I antigen during embryonic development. It is closely associated with the development and maturation of erythroid cells. Determines the expression of the blood group I antigen in erythrocytes. a beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->3)-beta-D-Gal-(1->4)-beta-D-GlcNAc derivative + UDP-N-acetyl-alpha-D-glucosamine = a beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->3)-[beta-D-GlcNAc-(1->6)]-beta-D-Gal-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + H(+) + UDP Protein modification; protein glycosylation. Isoforms A, B and C have different exons 1, but identical exons 2 and 3. Expressed in lens epithelium cells. Expressed in reticulocytes. Expression increases dramatically during development and oncogenesis. GCNT2 is involved in determining the blood group I system (Ii) [MIM:110800]. The i (fetal) and I (adult) antigens are determined by linear and branched poly-N-acetyllactosaminoglycans, respectively. A replacement during development of i by I is dependent on the appearance of a beta-1,6-N-acetylglucosaminyltransferase, the I-branching enzyme. The expression of the blood group I antigen in erythrocytes is determined by isoform C of GCNT2. The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycosyltransferase 14 family. N-acetyllactosaminide beta-1,6-N-acetylglucosaminyl-transferase