Model of the role of HDAC1 in Eg5-mediated mitotic progression. A) Eg5 (blue) is acetylated (Ac) and inactive prior to entry into mitosis. Upon entering the prophase of mitosis, HDAC1 (magenta) interacts with Eg5 after nuclear envelope breakdown, which results in Eg5 deacetylation, enhanced activity. Eg5 deacetylation promotes centromere separation and bipolar spindle formation (cell image), leading to metaphase progression and complete cell division. Associated proteins (orange and green) might be involved in the interaction. B) HDAC inhibitors prevent deacetylation of Eg5 by HDAC proteins. Acetylated Eg5 is inactive, which results in mitotic arrest due to monopolar spindle formation (cell image).