Rescue of defects in Runx1^−/− primitive erythrocytes by transgenic introduction of Runx1. (A) Structure of the G1-HRD-Runx1 (IE3.9int-Runx1) transgene. IE and II indicate exon IE and II, respectively. (B) Rescue of Ter119 expression on E10.5 Runx1^−/− primitive erythrocytes by introducing G1-HRD-Runx1 transgene. Black line, gray bold line, or red bold line represents Ter119 expression in Runx1^+/+, Runx1^−/−::G1-HRD-Runx1, and Runx1^−/−, respectively. Note that overexpression of Runx1 in Runx1^−/− erythrocytes induced slightly higher Ter119 expression. (C) Relative MFI values of Ter119 for Runx1^+/+ (normalized to MFI = 1), Runx1^−/−, and Runx1^−/−::G1-HRD-Runx1 embryos at E10.5. Bar graphs represents mean ratios plus or minus SD derived from Runx1^−/− (n = 4) and Runx1^−/−::G1-HRD-Runx1 (n = 4) embryos. Significant difference in Runx1^−/−::G1-HRD-Runx1 mice from Runx1^−/− mice was observed (*P < .001 by Student t test). (D) Rescue of abnormal morphology observed in Runx1^−/− primitive erythrocytes. May-Grunwald Giemsa stained cytospin preparation of peripheral blood cells from E12.5 embryos. Deformed shape observed in Runx1^−/− erythrocytes (Figure 1B) was rarely seen in Runx1^−/−::G1-HRD-Runx1 erythrocytes. Images in panels D were captured by Leica DC500 CCD camera with IM50 Imaging manager and a Leica DMRXA microscope using a 20×/0.50 NA objective.