DNA interstrand crosslinks (ICLs) present a formidable block to DNA metabolic processes and must be repaired for cell survival. While much work has been done to define the mechanism(s) of DNA interstrand crosslink repair in bacteria and yeast, very little is known about their repair in mammalian cells. It is has been proposed that both homologous recombination (HR) and nucleotide excision repair pathways are involved in DNA crosslink repair. It is becoming increasingly evident that there is overlap among the DNA repair pathways for certain types of DNA lesions. Our working hypothesis is that a number of proteins already defined as essential in DNA repair mechanisms are also required for efficient HR, or work together with HR in DNA interstrand crosslink processing and removal. The long-term objectives of the proposed research are to elucidate the molecular mechanism(s) involved in the removal of site-specific DNA interstrand crosslinks and complex DNA lesions (psoralen plus UVA radiation and mitomycin C crosslinks directed by triplex formation) from the mammalian genome, to identify interactions among the DNA repair pathways, and to determine the role(s) of homologous recombination, nucleotide excision, and mismatch repair mechanisms in the removal and processing of ICLs and complex DNA lesions. Specifically we propose to: 1) direct sitespecific DNA crosslinks to the genome in normal and recombination-deficient Chinese hamster ovary cells to elucidate the role of recombination in the removal of interstrand crosslinks and complex DNA lesions; 2) determine the mechanism(s) of removal of site-specific DNA interstrand crosslinks and complex DNA lesions directed by triplex-forming oligonucleotides in mammalian cell-free extracts; and 3) identify interactions between proteins involved in DNA repair pathways in the removal of site-specific DNA interstrand crosslinks and complex DNA lesions.