(A) Microtubule and DNA reorganization during mitosis and cytokinesis. Central spindle assembly during anaphase requires localization of centralspindlin a heterotetramer of MKLP1 and MgcRacGap/CYK-4, a GTPase activating protein. Compaction of the central spindle during telophase results in formation of the midbody, a process requiring centralspindlin. Normally, the midbody is cleaved in a process termed abscission, yielding 2 mononuclear cells. MKLP1 deficiency or aberrant MKLP1, as occurs in CDA III, results in cleavage furrow regression and ultimately binucleate cells. Subsequent rounds of failed cytokinesis could result in erythroid cells with up to 12 nuclei, as observed in CDA III. (B) Domain interactions of MKLP1 (adapted from White and Glotzer⁷ ). The linker region and coiled coil domains mediate MgcRacGAP interaction and promote oligomerization, respectively. Oligomerization promotes activity by enhancing interactions with microtubules. Interaction with 14-3-3 proteins inhibits cytokinesis by recruiting centralspindlin away from microtubules at the central spindle or midbody. This interaction is positively and negatively regulated by phosphorylation. Phosphorylation of S812 enhances activity by inhibiting interaction with 14-3-3, whereas S814 phosphorylation promotes 14-3-3 binding. Phosphorylation of S911 enhances MKLP1 activity by preventing its premature import into the nucleus during cytokinesis. Interaction with ARF6 GTPase enhances activity by competing with 14-3-3 binding. The P916R CDA III mutation described by Liljeholm et al, which impairs MLKP1 activity, is indicated at the C-terminus. Professional illustration by Kenneth X. Probst.