Cytokeratin 12 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 (MCD), an autosomal dominant disease that causes fragility of the anterior corneal epithelium.