The human blood group i and I antigens are characterized as linear and branched repeats of N-acetyllactosamine, respectively. Conversion of the i to the I structure requires the activity of I-branching -1,6-N-acetylglucosaminyltransferase (IGnT). Thus the blood group I gene is assigned to encode a -1,6-N-acetylglucosaminyltransferase; however, its identity has not been confirmed. The null phenotype of I, the adult i phenotype, provides a means to identify the I gene. Interestingly, the adult i phenotype has been noted to be associated with congenital cataracts in Asians.