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RhoA GTPase has been shown in vitro in cell lines and in vivo in nonmammalian organisms to regulate cell division, particularly during cytokinesis and abscission, when 2 daughter cells partition through coordinated actomyosin and microtubule machineries. To investigate the role of this GTPase in the rapidly proliferating mammalian erythroid lineage, we developed a mouse model with erythroid-specific deletion of RhoA. This model was proved embryonic lethal as a result of severe anemia by embryonic day 16.5 (E16.5). The primitive red blood cells were enlarged, poikilocytic, and frequently multinucleated, but were able to sustain life despite experiencing cytokinesis failure. In contrast, definitive erythropoiesis failed and the mice died by E16.5, with profound reduction of maturing erythroblast populations within the fetal liver. RhoA was required to activate myosin-regulatory light chain and localized at the site of the midbody formation in dividing wild-type erythroblasts. Cytokinesis failure caused by RhoA deficiency resulted in p53 activation and p21-transcriptional upregulation with associated cell-cycle arrest, increased DNA damage, and cell death. Our findings demonstrate the role of RhoA as a critical regulator for efficient erythroblast proliferation and the p53 pathway as a powerful quality control mechanism in erythropoiesis., (© 2015 by The American Society of Hematology.)