We have investigated DNA-mediated transfer of aminopterin resistance conferred by plasmid and UV resistance conferred by genomic DNA to the Chinese hamster ovary (CHO) cell line UV-135, a UVsensitive mutant defective in nucleotide excision repair. Plasmid pSV2gpt-CaPO4 coprecipitates induced aminopterin resistance with equal efficiency in the 6-thioguanine-resistant, aminopterin-sensitive, repairproficient parental line AA8-4(tg-1) and in . Genetic and molecular evidence for genomic DNAmediated transformation of UV-135(tg-2) cells with a putative excision repair gene were obtained by demonstrating that: (i) UV resistance transformation is dependent upon and specific for genomic DNA from excision repair-competent CHO cells: (ii) UV and drug coresistant colonies are bona fide transferants as verified by hybridization and Southern blotting analysis of pSV2gpt sequences in their genomic DNAs; (iii) confirmed transferants exhibit partial to near normal UV resistances for colony formation, and (iv) UVr transferants have near normal levels of excision repair capacity. The overall frequency of drug and UV resistance cotransformation was 8 x 10-8 per cell plated. This frequency was ca. 200-to 500-fold greater than that expected from coincident but independent UV' reversion and plasmid gene transfer events. DNA transfer techniques with this CHO system will be useful for further analysis of the essential structural DNA sequences, gene cloning, and expression of functional excision repair genes.Nucleotide excision repair (ER) is the major DNA metabolic pathway by which a variety of alkylation adducts, cross-links, and UV-induced photoproducts are eliminated from DNA (reviewed in reference 7). Genetic deficiency for repair of these damages is in part responsible for the propensity of most human xeroderma pigmentosum (XP) patients to develop skin cancers (21) and possibly for XPassociated degenerative changes in nonirradiated organ sites (10). Hence, XP and certain other genetic syndromes (17) correlate with altered ER capacity, alterations which can apparently increase cell susceptibility for certain initiation events in neoplasia. In none of these human syndromes has the normal products of these defective genes yet been identified.Molecular approaches are now available to identify. isolate, and manipulate expression of ER genes and polypeptides. In this work, we wanted to determine whether DNA transformation of repair-deficient cells would be useful for these purposes. The molecular identification of a possible human ER gene complementing the XP-A genotype has been reported previously (23). However, several technical difficulties with the use of XP cell lines for this type of experiment have made another mammalian cell system attractive for repair gene transfer.The availability of ER-proficient CHO cells and the technical advantages of genetic manipulation in this system led to the isolation of a large series of recessive, UV-sensitive (UV') mutants which fall into five complementation groups (4,(25)(26)(27). These mutan...