Recent genetic analyses in yeasts and biochemical studies in vertebrate cells have led to the discovery of a family of putative ATPases that play a fundamental role in chromosome condensation and segregation in mitosis. One of the members was also found to be involved in dosage compensation in Caenorhabditis elegans, providing a new link between global regulation of gene expression and chromosome structure. This unique family of proteins may control higher-order chromosome dynamics by regulated self-assembly or mechanochemical activity.