Part of the insoluble cornified cell envelope (CE) of stratified squamous epithelia. Directly or indirectly cross-linked to cornifelin (CNFN). Constituent of the scaffolding of the cornified envelope. Keratinocytes of epidermis and other stratified squamous epithelia. Substrate of transglutaminase. Specific glutamines or lysines are cross-linked to keratins, desmoplakin and to inter involucrin molecules (By similarity). Belongs to the involucrin family.