ABSTRACT The Ino80 chromatin remodeling complex plays an important role in the repair of radiation-induced DNA double strand breaks in lower eukaryotes. This discovery provides the first strong evidence that accessibility to DNA in the context of highly compact chromatin structure is a critical factor in DNA damage response. It becomes increasingly clear that maintenance of genome stability depends on highly coordinated actions of DNA damage repair, cell cycle checkpoint, and chromatin remodeling mechanisms. While the first two mechanisms have been the subject of extensive investigations during the past decades, the role of chromatin modification and remodeling in DNA damage response remains largely unclear, particularly in mammalian systems. Our proposed studies are aimed at delineating how chromatin remodeling activities support removal of DNA lesions and initiation of damage-induced cell cycle checkpoint signals. Our focus will be on two key subunits of the Ino80 ATP-dependent chromatin remodeling complex, Ino80 and Arp5. Ino80 is a unique member of the SNF2 superfamily that is believed to be a specialized chromatin remodeler assisting in DNA repair. Arp5 is an actin-related nuclear protein and an integral subunit of the Ino80 complex. Mutations of either gene in budding yeast render cells hypersensitive to a broad spectrum of genotoxic agents. In this application, we seek to understand how Ino80 modulates DNA damage responses. We have successfully created, via homologous targeting, loss-of-function human cellular models for INO80 and ARP5. These genetic model systems will serve as unique tools to study the function of Ino80 and Arp5 in cell proliferation, repair of ionizing radiation and UV-induced DNA lesions, damage-induced cell cycle arrest, and apoptosis. Our results are expected to further elucidate the mechanisms of the DNA damage response system and the molecular basis of genomic instability at large. Our results should also be useful for identification of novel therapeutic targets, especially targets for radiation sensitization. NARRATIVE Access to DNA lesions is a key prerequisite for many cellular mechanisms that act to protect the integrity of the human genome. Studies proposed in this application seek to understand the role of chromatin remodeling complex in creating such access by using molecular and genetics approaches. Results from the proposed work have the potential to unveil novel mechanism of genetic instability and to identify novel targets for cancer therapy.