Cross-linked envelope protein of keratinocytes. It is a keratinocyte protein that first appears in the cell cytosol, but ultimately becomes cross-linked to membrane proteins by transglutaminase. All that results in the formation of an insoluble envelope beneath the plasma membrane. Suprabasal layers of squamous-differentiated tissues such as epidermis, esophagus, tongue and trachea. Expressed during differentiation of squamous cells. During squamous differentiation of epidermal keratinocytes. This induction is repressed by retinoids. Belongs to the cornifin (SPRR) family.