Double-strand breaks (DSBs) occur frequently in the genome by DNA damaging agents or during genome replication. DSBs are hazardous because interaction between DNA ends from different double strand breaks can produce tumorigenic chromosome translocations. Little is known about how repair factors function in the context of chromatin and how translocations form in vivo. I developed a cell system in which a DSB can specifically be induced at a defined genomic site and follow the fate of damaged DNA in living cells. Using this system, I showed but that the broken ends are positionally stable and unable to roam the cell nucleus and I identified that the repair factor Ku80 is required for maintaining the alignment of broken ends. I extended the use of this system to probe how DSBs are recognized in vivo and how DDR pathways are triggered in the context of chromatin. I found that tethering a single component of a DSB repair complex to chromatin is sufficient to elicit the DDR in the absence of DNA damage and that stable association of the mediator of DDR MDC1 induces high local chromatin decondensation. I will use a combination of advanced live cell imaging and biochemistry techniques to further test the importance of nuclear architecture in maintaining genomic integrity and I will identify novel factors involved in DSB repair and alignment of broken chromosomes. More specifically I plan to: 1. Visualize the formation of chromosome translocations in vivo and determine the role of chromatin structure and nuclear organization in the process using time-lapse imaging. 2. Investigate the role of the repair factor MDC1 in chromatin decondensation using yeast two hybrid and advanced biochemistry in mammalian cells. 3. Identify novel proteins that accumulate in DSBs using biochemical fractionation and purification methods. 4. Identify novel proteins that align broken chromosome ends by high-through-put siRNA screening.