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The nuclei of human cells contain 2 meters of genomic DNA. How does it all fit? Compaction starts with the DNA wrapping around histone octamers to form nucleosomes, but it is unclear how these further compress into mitotic chromosomes. Ou et al. describe a DNA-labeling method that allows them to visualize chromatin organization in human cells (see the Perspective by Larson and Misteli). They show that chromatin forms flexible chains with diameters between 5 and 24 nm. In mitotic chromosomes, chains bend back on themselves to pack at high density, whereas during interphase, the chromatin chains are more extended.