Oligonucleotide-directed gene alteration produces a targeted deoxyribonucleic acid (DNA) sequence change in the genome of mammalian cells at low frequency that is only detectable by highly sensitive methods. To measure the low frequency, we have established an assay using the mutant lacZ vector that contains a single point mutation in the lacZ gene, which results in a loss of enzymatic activity. When cells containing this mutant reporter gene are corrected by gene targeting, the mutant beta-galactosidase enzymatic activity is restored, and corrected cells can be visualized by histochemical staining. Using this method, we detected a low level of gene correction in the primary human keratinocytes, in spite of highly efficient nuclear uptake of oligonucleotide. Therefore, it is important to consider many other factors for successful gene repair, including DNA repair and recombination activities, status of replication and transcription, in addition to the well-known requirements like the quality and delivery of oligodeoxy nucleotides to cells. Available methods to manipulate epidermal stem cells and the accessibility of the tissue make the epidermis attractive for gene targeting. Given the low frequency, however, general selection procedures and amplification of corrected cells via epidermal stem cells are ultimately needed to make the gene repair technology practical.