ERCC4 is an essential human gene in the nucleotide excision repair (NER) pathway, which is responsible for removing UV-C photoproducts and bulky adducts from DNA. Among the NER genes, ERCC4 and ERCC1 are also uniquely involved in removing DNA interstrand cross-linking damage. The ERCC1-ERCC4 heterodimer, like the homologous Rad10-Rad1 complex, was recently found to possess an endonucleolytic activity that incises on the 5 side of damage. The ERCC4 gene, assigned to chromosome 16p13.1-p13.2, was previously isolated by using a chromosome 16 cosmid library. It corrects the defect in Chinese hamster ovary (CHO) mutants of NER complementation group 4 and is implicated in complementation group F of the human disorder xeroderma pigmentosum. We describe the ERCC4 gene structure and functional cDNA sequence encoding a 916-amino-acid protein (104 kDa), which has substantial homology with the eukaryotic DNA repair and recombination proteins MEI-9 (Drosophila melanogaster), Rad16 (Schizosaccharomyces pombe), and Rad1 (Saccharomyces cerevisiae). ERCC4 cDNA efficiently corrected mutants in rodent NER complementation groups 4 and 11, showing the equivalence of these groups, and ERCC4 protein levels were reduced in mutants of both groups. In cells of an XP-F patient, the ERCC4 protein level was reduced to less than 5%, consistent with XPF being the ERCC4 gene. The considerable identity (40%) between ERCC4 and MEI-9 suggests a possible involvement of ERCC4 in meiosis. In baboon tissues, ERCC4 was expressed weakly and was not significantly higher in testis than in nonmeiotic tissues.Nucleotide excision repair (NER) is a universal DNA repair pathway that acts on most bulky chemical adducts and the major photoproducts (cyclobutane and [6-4] pyrimidine dimers) produced by UV radiation (19,25,52,68). In xeroderma pigmentosum (XP) patients, who have a very high risk of skin cancer due to solar exposure, there is a partial or complete deficiency in NER (15). This observation provided a critical link between deficiency of an error-free repair process and enhanced mutagenesis and carcinogenesis in humans (15,37). Genetic analysis of XP cell lines identified seven complementation groups of incision-defective XP patients (30, 79) and a common variant form of XP that is excision proficient (14). Parallel studies with rodent cell mutants isolated in culture demonstrated 11 complementation groups that are phenotypically similar to the XP lines (51). The rodent mutants proved well suited for cloning NER genes on the basis of functional complementation because of efficient DNA transformation (74,76,81,83), and considerable overlap was found between the XP and rodent groups (68). Thus, the genes XPA (66), XPB (ERCC3) (83), XPC (34), XPD (ERCC2) (18, 81), and XPG (ERCC5) (36, 57) have been characterized, as has the ERCC1 NER gene (76), which is not involved in any known human disorder (77).Recently the NER process has been reconstituted by using recombinant proteins and highly purified factors (1,43,44).With the reconstituted systems, insights in...