Chromatin has shifted into the focus of attention as a key to understanding the regulation of nuclear processes such as transcription. Protein machines have been described that use the energy of ATP to render chromatin dynamic and hence active, but which may also be involved in chromatin assembly. The discovery of three different Drosophila nucleosome remodeling complexes that contain imitation switch (ISWI), an ATPase with a high degree of sequence conservation from yeast to human, points to a central function of this ATPase in chromatin dynamics.