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. Not detected in normal lung tissue but seen in tumor tissues. Cells around the keratin pearls contain high levels. During squamous differentiation of epidermal keratinocytes. Belongs to the cornifin (SPRR) family.