Erythroid phenotype of PU.1G/G fetuses. (A) Representation of targeted PU.1 allele. Black boxes indicate exons (exon I not to scale). Exon numbers are indicated by roman numerals. RT-PCR primers (P1-P4) are shown at the top. The DNA fragment present in the targeting vector is depicted with a thicker line in the WT allele. A and B are the probes used for Southern blot analyses; E, EcoRI; S, SpeI; IVS, intron from the rabbit β-globin gene. (B) Altered erythropoiesis in PU.1G/G FL. The 12.5- and 16.5-dpc FL cells were analyzed for TER119 expression by flow cytometry. Bar graphs display the average number of TER119⁺ cells per FL, calculated from 6 mutant and 6 WT fetuses for each stage. (C) Quantification of immature (CFC-mix/ery; left panel) and mature (CFU-E; right panel) erythroid precursors at different stages of PU.1G/G and WT FL development. Means and SDs were obtained using more than 3 fetuses of each genotype from 2 different experiments, per stage of development.