AbstractCongenital dyserythropoietic anemias (CDAs) are rare disorders characterized by morphologic abnormalities of erythroid precursors leading to ineffective erythropoiesis. CDA type III (CDAIII), characterized by erythroblast multinucleation, represents the rarest form with only ∼60 patients described in the literature. Previous work, studying two independent families, identified a causative dominant missense mutation in KIF23, which encodes for the kinesin MKLP1. Here, we describe a sporadic CDAIII case associated with compound heterozygous variants in RACGAP1, a gene not previously associated with any disease. RACGAP1 encodes CYK4, a GTPase activating protein (GAP) for Rho-family GTPases, which interacts with MKLP1 to form the centralspindlin complex. Functional assays show these RACGAP1 variants cause cytokinesis defects due, at least in part, to altering the substrate specificities of the GAP activity of CYK4. These findings provide novel insights into the structural determinants of the GAP activity and demonstrates that cytokinesis failure due to centralspindlin defects leads to CDAIII. Our findings highlight the importance of viewing diseases as malfunctions of common biological pathways/complexes and suggests that next-generation sequencing analysis pipelines should integrate a systems approach in order to identify such functionally related variants.