The second family of SWI-like ATP-dependent chromatin-remodelling complexes is the ISWI family. These complexes were first identified in D. melanogaster, which has a single ISWI ATPase. This ATPase is the core component of three types of ISWI complex: NURF (nucleosome-remodelling factor), ACF (chromatin-assembly factor) and CHRAC (chromatin accessibility complex) complexes (see ref. 51 for a review). Loss-of-function mutations in Iswi are lethal during late pupal or larval development8, perhaps as a result of impaired expression of homeotic genes in the imaginal discs. Restricted expression of an ATPase-dead, dominant-negative allele of Iswi leads to defects in organogenesis, owing to its widespread role in cell viability and cell division8. More intriguingly, ISWI complexes are involved in regulating higher-order chromatin structure. This is evident from a study showing that dominant-negative Iswi or Iswi loss-of-function mutations results in marked global decondensation of mitotic chromosomes, owing to the apparent requirement for ISWI in incorporating the linker histone protein H1 (ref. 52), which is in turn required for normal chromosome condensation and compaction53. In females, ISWI deficiency leads to complete sterility8. This results from the misregulation
