Involucrin is a precursor of the keratinocyte cornified envelope that is specifically expressed in the suprabasal layers of the epidermis and other stratifying squamous epithelia. To study involucrin gene expression and the function of involucrin, we expressed a 6 kb DNA fragment of the human involucrin gene, containing approximately 2.5 kb of upstream sequence and 0.5 kb of downstream sequence, in transgenic mice. The transgene produces a 68 kDa protein that is detected by a human involucrin-specific antibody, and is expressed in a tissue-specific and differentiation-appropriate manner (i.e., expression is confined to the suprabasal layers of the epidermis, extocervix, trachea, esophagus and conjunctiva). Soluble involucrin levels are two to four times higher in transgenic epidermal keratinocytes compared to human foreskin keratinocytes. Newborn heterozygous animals have a normal birth weight and a normal appearing epidermis and hair growth begins at 4 to 5 days of age (i.e., the same time as hair growth in non-transgenic animals). In a subpopulation of the newborn homozygous animals birth weight is reduced, the epidermis is scaly and hair growth begins late, at around 9 to 10 days of age. In addition, the hair tends to stand erect on both heterozygous and homozygous adult animals giving the appearance of diffuse alopecia. Immunofluorescent and electron microscopy localize involucrin in the hair follicle and cornified envelope, respectively. These results suggest that overexpression of involucrin may cause abnormalities in hair follicle structure/function and cornified envelope structure. These animals provide a new model for the study of cornified envelope structure and function.