The introduction and expression of a foreign gene provide a powerful tool for investigating functions and regulation of a gene of interest; however, keratinocytes have a major drawback in that foreign genes are hardly transfected by conventional methods and stable transformants are most difficult to establish in normal keratinocytes with a limited short life span. To overcome these problems, we used an adenovirus vector, Ax, developed by Saito et al, which yields desired recombinant viruses at an efficiency about 100-fold that of conventional methods, and by which genes are expressed at a high level under the control of a composite CAG promoter. We established Ax vectors carrying various isoforms of protein kinase C (PKC). Using these vectors, we found that the α and ζ isoforms of PKC, but not the η and δ isoforms, mediate terminal differentiation in normal human keratinocytes. These Ax-vectors are also applicable to organ culture of mouse embryos. Advantages and disadvantages of adenovirus vectors and their use for keratinocyte biology are reviewed.