Live imaging of DNA loop extrusion To spatially organize chromosomes, ring-shaped protein complexes including condensin and cohesin have been hypothesized to extrude DNA loops. Condensin has been shown to exhibit a DNA-translocating motor function, but extrusion has not been observed directly. Using single-molecule imaging, Ganji et al. visualized in real time a condensin-mediated, adenosine triphosphate-dependent, fast DNA loop extrusion process. Loop extrusion occurred asymmetrically, with condensin reeling in only one end of the DNA. These data provide unambiguous evidence of a loop extrusion mechanism for chromosome organization. Science , this issue p. 102