Cytokeratin 3 is a member of the intermediate filament family of proteins and is a heterotetramer of two type I and two type II keratins. Keratin 3 is specifically expressed in the corneal epithelium with family member KRT12. Cytokeratin 12 encodes the type I intermediate filament chain keratin 12, expressed in corneal epithelia. Defects in KRT3 and KRT12 are a cause of Meesmann corneal dystrophy, an autosomal dominant disease that causes fragility of the anterior corneal epithelium. Symptoms occur in adulthood and include rupture of the corneal microcysts that may lead to photophobia, contact lens intolerance and intermittent diminution of visual acuity.