The evolutionarily conserved SWI-like ATP-dependent chromatin-remodelling complexes can be broadly divided into four main families on the basis of the sequence and structure of the ATPase subunit: SWI/SNF, ISWI, CHD and INO80 complexes. However, many of the predicted SWI/SNF-like ATPases do not fit any of these classes and await characterization. Why does the regulation of a genome require so many functionally non-redundant ATP-dependent chromatin remodellers if they all act to increase nucleosome mobility? Emerging evidence supports at least two possible explanations. First, new roles and molecular functions of chromatin remodellers have been discovered recently. For example, ISWI complexes have been shown to be required for maintaining the higher-order structure of the D. melanogaster male X chromosome8, and INO80 complexes are involved in telomere regulation, chromosome segregation, and checkpoint control and DNA replication during cell division (see ref. 9 for a review). Hence, it is becoming clear that SWI-like remodellers are intricately involved in many aspects of cell biology beyond transcription. Second, even within their traditional role of transcriptional regulation, ATP-dependent chromatin remodellers do not function in a consistent manner. Brahma-associated
