PURPOSE. Cell division of corneal limbal stem cells gives rise to transient amplifying cells that ultimately differentiate to form the multilayered corneal epithelium. The mechanisms that regulate changes in gene expression during this process are not well understood. In the present study, the involucrin gene was used as a model to study this regulation. METHODS. Regulation of human involucrin gene expression and promoter activity was assessed using in vivo transgenic mouse models and cultured primary human corneal epithelial cells. RESULTS. Human involucrin (hINV) is a structural protein that is selectively expressed in differentiating corneal epithelial cells. The results reveal that an activator protein one (AP1) DNAbinding site is essential for appropriate basal and stimulusdependent hINV promoter activity. Mutation of this site, AP1-5, results in a loss of hINV gene expression in the corneal epithelium in vivo and in cultured corneal epithelial cells. A gel mobility supershift analysis revealed interaction of the AP1 factors, Fra-1 and JunB, with this element. Inhibition of AP1 function with a dominant-negative form of AP1 also inhibited expression. Treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C activator, increased hINV gene expression, a response that correlates with increased AP1 factor (Fra-1 and JunB) binding to the hINV gene AP1-5 response element. CONCLUSIONS. These findings point to an essential role for AP1 transcription factors, acting through a distal regulatory region AP1-5 element, in the regulation of involucrin gene expression during corneal epithelial cell differentiation. (Invest Ophthalmol Vis Sci.