Eukaryotic genomes are large compared to the cells they are contained in, consisting of 2 meters DNA in humans and 20 or more meters in some species such as salamanders. At the various stages of the lifetime of cells, their genomes need to be transcribed, replicated, repaired, recombined, condensed and segregated with high speed and precision. For many of these processes it is crucial that the genome is folded correctly. A key molecule mediating this genome organization is cohesin, a large ring-shaped ATPase complex initially discovered for its essential role in sister chromatid cohesion and chromosome segregation. Our work indicates that in addition to mediating cohesion in proliferating cells, cohesin has a universal role in all cells in forming chromatin loops. Whereas cohesin is thought to mediate cohesion as a passive topological linker, several observations imply that cohesin forms chromatin loops actively by a mysterious extrusion mechanism. The major aims of this project are to understand how cohesin interacts with DNA to perform its functions in sister chromatid cohesion and chromatin organization and to obtain insight into the mechanism by which cohesin forms chromatin loops. Addressing these questions will contribute to understanding genome organization, function and inheritance, and may help to explain why cohesin subunits are among the most frequently mutated tumor suppressor genes in human cancers.