In mammalian nucleotide excision repair (NER), the ERCC1 protein is known to act as a complex with ERCC4 (XPF) protein, which is necessary for stability of ERCC1, and this complex introduces an incision on the 5' side of a damaged site in DNA. ERCC1 also binds to XPA protein to make a large protein complex at the site of DNA damage. Since no human disease associated with ERCC1 has been identified, Chinese hamster ovary (CHO) cell lines defective in ERCC1 are a unique source for characterization of ERCC1 deficiency in mammalian cells. We have isolated the full length ERCC1 cDNA from a wild-type CHO cell line and analyzed mutations in two CHO cell lines which fall into complementation group 1 of UV-sensitive rodent cell lines. One cell line, 43-3B, has a missense mutation at the 98th residue (V98E). The in vitro translated mutant protein of 43-3B is unable to bind to XPA protein. Although the mutant protein is able to bind to XPF protein in vitro, the mutant protein is highly unstable in vivo. These defects presumably cause the NER deficiency of this cell line. Another mutant, UV-4, has an insertion mutation in the middle of the coding sequence, resulting in a truncated protein due to a nonsense codon arising from the frameshift. Thus, these two mutant cell lines are deficient in the function of the ERCC1 gene for NER.