Cohesin extrudes DNA loops DNA is folded into loops in eukaryotic cells by a process that depends on a ring-shaped adenosine triphosphatase complex called cohesin. Davidson et al. and Kim et al. now show that in the presence of the NIPBLMAU2 protein complex, the human cohesin complex can function as a molecular motor that extrudes DNA loops with high speed in vitro. In contrast to how it mediates sister chromatid cohesion, cohesin does not appear to entrap DNA topologically during loop extrusion. The results provide direct evidence for the loop extrusion model of chromatin organization and suggest that genome architecture is highly dynamic. Science , this issue p. 1338 , p. 1345