Adult i phenotype is a rare autosomal recessive condition that was found to be highly associated with congenital cataract. The I and i antigens are carbohydrate structures on glycoproteins and glycolipids on the cell surface of a variety of tissues and body fluids. Most adult red blood cells (RBCs) abundantly express I antigen. Conversion of the i antigen into an I structure takes place during the first 18 months after birth as a result of the expression of a specific transferase, I-branching GCNT2. Lack of this enzyme results in the adult i phenotype.
