Transglutaminases, including the product of the TGM1 gene, catalyze formation of epsilon-(gamma-glutamyl)-lysine crosslinks in proteins and stabilize biological structures. In the epidermis, TGM1 is required for the formation of the cross-linked envelope. Point mutations in the TGM1 gene cause deficits in enzyme activity. TGM1 is normally expressed in the suprabasal cells of stratifying epithelia such as epidermis, the upper digestive tract, the female lower genital tract and in the endometrial epithelium late in pregnancy. It is also expressed as a result of squamous metaplasia in the trachea induced by vitamin A deprivation and in a number of epithelial cell types, including those from bladder and endometrium, induced by culture on plastic.