Analysis of primitive erythroid progenitors and differentiated cells in Runx1^−/− embryo. (A) Primitive erythroid colonies from E8.5 Runx1^+/− yolk sacs (n = 3) and Runx1^−/− yolk sacs (n = 3). (B,C) Benzidine staining of E8.5 embryos. Hemoglobinized cells were observed inside blood vessels of yolk sac. Similar staining levels were obtained for wild-type and Runx1^−/− embryos. (D) Flow cytometric analysis of E9.5 yolk sac using anti-Ter119 antibody. Black line, gray bold line, or red bold line represents Ter119 expression in Runx1^+/+, Runx1^+/−, or Runx1^−/−, respectively. (E) Relative MFI values of Ter119 for Runx1^+/+ (normalized to MFI = 1), Runx1^+/−, and Runx1^−/− embryos at E9.5 and E10.5. Bar graphs represents mean ratios plus or minus SD derived from Runx1^+/− (n = 3 E9.5, n = 4 E10.5) and Runx1^−/− (n = 2 E9.5, n = 4 E10.5) embryos. Significant differences in Runx1^−/− mice from Runx1^+/− mice were observed (**P = .017, *P = .039 by Student t test). Original magnifications ×16. Images in panel B were captured with a Leica MZ FLIII microscope and a Leica DC500 CCD camera with IM50 Imaging Manager, with a PLAN APO 1.0 lens (1.0×/0.125 NA).