Erythropoietic differentiation in Rcor1 knockout mice is blocked at the transition from proerythroblast to basophilic erythroblast. (A) Flow cytometry profiles of fetal liver cells stained with CD71 and TER119. R0-R5: Gates of different erythroblast populations according to their expression levels of CD71 and TER119. R0 contains mixed populations of hematopoietic stem cell (HSC) and early progenitors, such as CMP, GMP, and MEP; R1 consists of mostly immature RBC progenitors, including BFU-E and CFU-E; R2 comprises mainly proerythroblasts and early basophilic erythroblasts; R3 contains early and late basophilic erythroblasts; R4 is composed of chromatophilic and orthochromatophilic erythroblasts; and R5 contains late orthochromatophilic erythroblasts and reticulocytes. Note that the transition from R2 to R3 is arrested in the mutant fetal liver. (B) May-Grunwald Giemsa staining showing similar morphology of FACS-sorted mutant and control E14.5 fetal liver cells. Scale bar: 20 μm. (C) Schematic diagram showing transplant of fetal liver cells that express CD45.2 and β-globin haplotype Hbbs (donor) into irradiated mice double congenic for CD45.1 and β-globin haplotype Hbbd (host). (D) Donor cell contribution to circulating leukocytes of adult WT mice transplanted with 2 million E13.5 mutant or control fetal liver cells. (E) Hemoglobin electrophoresis analysis indicates that mutant fetal liver cells cannot generate RBCs after transplantation into WT adult mice. (F) Immunostaining for Rcor1 protein in FACS-sorted E14.5 fetal liver cells. Mutant R0 cells serve as a negative control (NC). Nuclei labeled with 4′,6 diamidino-2-phenylindole (DAPI). Scale bar: 20 μm. Images in this figure were acquired with a Zeiss Axiovert S-100, a Zeiss plan-neofluar ×63/1.25 oil lens, and an AxioCam HRc camera. D, donor; H, host.