Cell strains derived from patients having a hereditary disorder associated with defects in repair of DNA damage such as xeroderma pigmentosum and mutants isolated from established rodent cell lines provide the tools for genetic and biochemical analysis of DNA repair pathways in mammalian cells. Complementation studies using these cells have illustrated the genetic and biochemical complexity of these pathways. The precise nature of the genes and gene products involved in these mutants has not yet been resolved. Isolation of repair genes by recombinant DNA technology would open up new approaches to the elucidation of repair mechanisms in mammalian cells. Here we report the molecular cloning of a human repair gene (ERCC1) that complements the repair defect in a Chinese hamster ovary (CHO) mutant cell line.