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. It is most likely that this mechanism ensures the ordered action of the two complexes, namely, condensin II first and condensin I later. On metaphase chromosomes, condensins I and II are both enriched in the central axis in a non-overlapping fashion (Figure 5). Depletion experiments in vivo and immunodepletion experiments in Xenopus egg extracts demonstrate that the two complexes have distinct functions in assembling metaphase chromosomes. Cells deficient in condensin functions are not arrested at a specific stage of cell cycle, displaying chromosome segregation defects (i.e., anaphase bridges) and progressing through abnormal cytokinesis. The relative contribution of condensins I and II to mitosis varies among different eukaryotic species. For instance, each of condensins I and II plays an essential role in embryonic development in mice. They have both overlapping and non-overlapping functions during the mitotic cell cycle. On the other hand, condensin II is non-essential for mitosis in the primitive alga C. merolae and the land plant A. thaliana. Curiously, condensin II plays a dominant role over condensin I in the C. elegans early embryos. This peculiarity could be due to the fact that C. elegans has a specialized chromosome structure known as holocentric chromosomes. Fungi, such as S. cerevisiae and S. pombe have no condensin II from the first. These differences among eukaryotic species provide important implications in the evolution of chromosome architecture (see the section of "Evolutionary implications" below). It has recently become possible that cell cycle-dependent structural changes of chromosomes are monitored by a genomics-based method known as Hi-C (High-throughput chromosome conformation capture). The impact of condensin deficiency on chromosome conformation has been addressed in budding yeast, fission