Proper chromatin condensation and sister chromatid resolution are essential for the maintenance of chromosomal integrity during cell division, and is in part mediated by a conserved multisubunit apparatus termed the condensin complex. The core subunits of the complex

Chromatin condensation is a fundamental cellular process in which entangled interphase chromatin fibers are actively resolved and packaged into physically discrete and compact chromosomes that undergo segregation during cell division. This complex series of events enables the cell to proceed through mitosis and faithfully distribute genetic information to each daughter cell. The inability to execute this process correctly can result in catastrophic chromosome segregation defects as is observed in mutants defective in topoisomerase II (DiNardo et al., 1984). Failure to disentangle chromatin fibers prevents compaction necessary for mitotic chromosome condensation (Gimenez-Abian et al., 2000; Cuvier and Hirano, 2003). In addition to topoisomerase II, other conserved proteins and histone modifications play roles in chromosome condensation (Hirano, 2002). The SMC proteins are evolutionarily conserved across phylogenetic kingdoms with representatives in both prokaryotes and eukaryotes. The eukaryotic SMC proteins can be subdivided into six distinct subfamilies (SMC1-6) (Hirano, 2002; Jessberger, 2002), and three functional categories can be defined based on the formation of their heterodimers. The SMC2/SMC4 heterodimers assemble with three other nonSMC proteins to form the ‘condensin’ complex, which is involved in mitotic chromosome condensation and dosage compensation (Hirano and Mitchison, 1994; Lieb et al., 1998; Sutani et al., 1999; Kimura et al., 2001). Members of the SMC1 and SMC3 families form heterodimers to give rise to the ‘cohesin’ complex, which is involved in sister chromatid cohesion (Michaelis et al., 1997; Losada et al., 1998). Lastly, SMC5 and SMC6 members form heterodimers that are involved in DNA recombination/repair (Mengiste et al., 1999; Fousteri and Lehmann, 2000). All SMC proteins consist of five distinct domains: nucleotide-binding domains located at the N and C termini lie adjacent to two coiled-coil regions, which are separated by a hinge region (Hirano, 2002; Jessberger, 2002). The catalytic function of the SMC subunits is regulated by nonSMC members of these multisubunit complexes. Mutations in genes encoding components of SMC complexes result in a variety of defects, ranging from lethality to developmental abnormalities (Saka et al., 1994; Strunnikov et al., 1995; Bhat et al., 1996; Lieb et al., 1998; Freeman et al., 2000; Steffensen et al., 2001; Bhalla et al., 2002; Hagstrom et al., 2002). 3283 Development 130, 3283-3295 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00542