Carbohydrate sulfotransferase 6 (CHST6) catalyzes the transfer of sulfate to position 6 of non-reducing ends of N-acetylglucosamine (GlcNAc) residues on keratan sulfate (KS). KS plays a central role in maintaining corneal transparency. Defective CHST6 (Nakazawa et al. 1984) results in unsulfated keratan deposited within the intracellular space and the extracellular corneal stroma leading to macular dystrophy, corneal type I (MCDC1; MIM:217800). MCDC1 is an early-onset, ocular disease characterized by bilateral, progressive corneal opacification, and reduced corneal sensitivity (Jones & Zimmerman 1961). MCD can be subdivided into 2 types on the basis of immunohistochemical studies and serum analysis for keratan sulfate; MCD type I, in which there is a virtual absence of sulfated KS-specific antibody response in the serum and cornea and MCD type II, in which the normal KS-specific antibody response is present in cornea and serum (Yang et al. 1988).