Involved in corneal epithelium organization, integrity and corneal keratin expression. Heterotetramer of two type I and two type II keratins. Keratin-3 associates with keratin-12. Expressed in the corneal epithelium (at protein level) (PubMed:7508359, PubMed:8977471, PubMed:26758872). Also expressed in the suprabasal limbal epithelium of the cornea (at protein level) (PubMed:7508359). Expression commences in corneal peridermal epithelium in the embryo at 15.5 dpc (PubMed:16431949). Between 15.5 dpc and postnatal day 4 (P4), expression is restricted to the suprabasal and/or superficial cells when the corneal epithelium begins to stratify (PubMed:7508359, PubMed:16431949). At P30 expression is sporadically detected in the basal corneal epithelium and the number of positive basal cells increases as the mice grow older (PubMed:16431949). Expression is retarded by epidermal growth factor (EGF). Knockout mice are fertile and morphologically normal with no defects in the skin or hair (PubMed:8977471). Corneal epithelia show histological abnormalities and can be easily detached from the corneal surface (PubMed:8977471). Reduced number of cell layers in the corneal epithelium, with superficial epithelial cells showing reduced adherence to underlying cell layers (PubMed:8977471). Keratin intermediate filaments aggregate into dense bundles in the corneal epithelium, with a reduced number of keratin filaments in basal and suprabasal epithelial cells, and an absence of keratin filaments in superficial epithelial cells (PubMed:8977471). There are two types of cytoskeletal and microfibrillar keratin: I (acidic; 40-55 kDa) and II (neutral to basic; 56-70 kDa). Belongs to the intermediate filament family.