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A construct containing -2500 base pairs (bp) of 5' upstream and -700 bp of 3' downstream sequence was used to drive the expression of an intronless human K14 gene in vitro and in vivo. To track the expression of the gene, a small sequence encoding the antigenic portion of neuropeptide substance P was inserted in frame 5' to the TGA translation stop codon of the gene. Surprisingly, this gene was expressed promiscuously in a wide variety of cultured cells transiently transfected with the construct. In contrast, when introduced into the germ line of transgenic mice, the construct was expressed in a fashion analogous to the endogenous K14 gene-namely, in the basal layer of stratified squamous epithelia. Our results suggest that some regulatory mechanism is overridden as a consequence oftransient transfection but that sequences that can control proper K14 expression are present in the construct. The appropriate tissue-specific and differentiation-specific expression of K14 P in transgenic mice is an important first step in characterizing a promoter that could be employed to drive the foreign expression of drug-related genes in the epidermis of skin grafts.Among the major proteins exclusive to epithelial tissues are keratins, a family of >20 proteins whose members are differentially expressed in a tissue-, differentiation-, and developmental-specific fashion (for review, see ref. 1). Based on sequence homologies, these proteins can be subdivided into two distinct groups: type I keratins are smaller and acidic (pKi = 4.5-5.5), and type II keratins are larger (53-67 kDa) and more basic (pKi = 5.5-7