Ionizing radiation (IR), such as X- or ?- rays, has been demonstrated to induce more sublethal base-damage and single strand breaks (SSBs) in comparison to DNA double strand breaks (DSB). This sublethal damage is a very potent inducer of replication blockage, despite being less effective in cell killing than DSBs. It is anticipated hat the molecular factors that modulate the effectiveness of cells to cope with the replication blockage plays a major role in the susceptibility of cells to IR-induced genomic instability. A mechanistic understanding on how replication blockages are resolved and the consequences of compromised resolution of replication blocks are critical to understand IR-induced carcinogenesis. This study will focus on the contribution of impaired resolution of replication blocks to genomic instability due to BCCIP and homologous recombination (HR) defects. The BCCIP gene is known as an important accessory factor that modulates HR, a pathway of critical importance to resolve replication blocks. Subtle down-regulation of BCCIP is sufficient to induce genomic instability and tumorigenesis. BCCIP loss of function is also associated with poor outcome of radiation therapy, and aneuploidy and polyploidy, feature that are associated with cancer aggressiveness. New preliminary studies suggest that: 1) a BCCIP variation is associated with secondary or recurrence cancers after radiation therapy; 2) BCCIP interacts with PCNA through a PIP motif; 3) BCCIP deficiency synergizes with moderate dose of IR to induce sister chromatid union (SCU) that gives rises to anaphase bridges and mitotic errors; 4) BCCIP deficiency increases the ultra-fine DNA bridges between sister chromatids during mitosis; and 5) the BCCIP? isoform is associated with mitotic spindles. We hypothesize that the two isoforms of human (BCCIP? and BCCIP) proteins control multiple aspects of chromosome integrity: interphase BCCIP (nuclear) functions in replication to ensure structural integrity of chromosomes, while the mitotic (cytoplasmic) BCCIP has a predominant role in microtubule dynamics. The coordinated roles of BCCIP in both suppressing structural chromosome damage and in regulating microtubule dynamics may hold a key to understand why a subtle or partial loss of BCCIP function is sufficient to trigger tumorigenesis, and is associated with radiation-induced cancer. Aim 1 will identify the function of BCCIP isoforms and the role of BCCIP-PCNA interaction in resolving replication blockages induced by DNA damage including low dose and dose rate IR. Aim 2 will define the physical nature of hazardous chromosomes structures that are synergistically induced by both BCCIP deficiency and sublethal dose of IR exposure in order to understand the mechanisms by which replication obstacles contribute to mitotic errors. Aim 3 will characterize a new function of BCCIP in mitotic spindles that coordinates the mitotic apparatus and DNA replication in interphase to safeguard mitotic fidelity. Many therapeutic and environmental agents, including IR, initially cause DNA damage that hinder DNA replication in interphase, which later on, can have dire consequences in mitosis. Our studies provide a rare opportunity to understand how replication errors contribute to mitotic defects, and how BCCIP serves as a molecular modulator for IR induced genomic instability.