It has been proposed that condensin I anchors DNA between Ycg1-Brn1 subunits and pulls DNA asymmetrically to form large loops. Moreover, it has been shown that condensin complexes can traverse each other, forming dynamic loop structures and changing their sizes. It is unknown how condensins might act on nucleosomal DNA. Recent development of a reconstitution system has identified the histone chaperone FACT as an essential component of condensin I-mediated chromosome assembly "in vitro", providing an important clue to this problem. It has also been shown that condensins can assemble chromosome-like structures in cell-free extracts even under the condition where nucleosome assembly is largely suppressed. This observation indicates that condensins can work at least in part on non-nucleosomal DNA in a physiological setting. Only limited information is currently available as to the functional contribution of individual subunits of condensins to their activities. An SMC2-SMC4 dimer has an ability to reanneal complementary single-stranded DNA. This activity does not require ATP. For eukaryotic complexes, it has been reported that HEAT repeat subunits contribute to part of DNA binding and to the assembly of chromosome axes. Flexible and extendable nature of HEAT repeats could underlie the dynamic action of condensins and the architecture of mitotic chromosomes. Several attempts on mathematical modeling and computer simulation of mitotic chromosome assembly, based on molecular activities of condensins, have been reported. Representative ones include modeling based on loop extrusion, stochastic pairwise contacts and a combination of looping and inter-condensin attractions. In human tissue culture cells, the two condensin complexes are regulated differently during the mitotic cell cycle (Figure 4). Condensin II is present within the cell nucleus during interphase and participates in an early stage of chromosome condensation within the prophase nucleus. On the other hand, condensin I is present in the cytoplasm during interphase, and gains access to chromosomes only after the nuclear envelope breaks down (NEBD) at the end of prophase. During prometaphase and metaphase, condensin I and condensin II cooperate to assemble rod-shaped chromosomes, in which two sister chromatids are fully resolved. Such differential dynamics of the two complexes is observed in "Xenopus" egg extracts, mouse oocytes, and neural stem cells, indicating that it is part of a fundamental regulatory mechanism conserved among different organisms and cell types.