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. Some glutamines and lysines are cross-linked to other involucrin molecules, to other proteins such as keratin, desmoplakin, periplakin and envoplakin, and to lipids like omega-hydroxyceramide. Belongs to the involucrin family.