The i and I antigens of the I blood group system are carbohydrate structures carried on glycolipids and glycoproteins and are characterized as straight or branched glycochains composed of repeating N-acetyllactosamine (LacNAc) units, respectively. Conversion of i antigen into an I-active structure requires the activity of the I-branching enzyme, beta-1,6-N-acetylglucosaminyltransferase, which adds the decisive GlcNAc-beta-1-6 branch onto the straight poly-LacNAc chains. Expression of the i and I antigens on red blood cells (RBCs) is reciprocal and developmentally regulated. Adult human RBCs predominantly express I antigen, whereas fetal and neonatal RBCs predominantly express i antigen. After birth, I antigen levels increase gradually as i antigen levels fall, with the normal Ii status of adult RBCs reached after about 13 to 20 months. Mutations that specifically affect 1 of the 3 variants produced by the GCNT2 gene cause the rare adult i phenotype, in which adult RBCs are rich in i antigen and contain low levels of I antigen. Mutations that eliminate all 3 GCNT2 variants cause the adult i phenotype with congenital cataract. [From OMIM]