Enforced expression of FOG-1 blocks mast cell development. (A) FACS expression analysis for eGFP, Gr-1, CD11b, or FcεRI in GMPs transduced with retroviruses expressing eGFP alone (vector) or FOG-1–IRES-GFP (FOG-1), and cultured at limiting dilution in the presence of SCF, IL-3, IL-6, and IL-9 for 3 wk. Percentages of total cells that fall within the boxed FACS gates are shown. May-Grunwald-Giemsa stains of the GFP⁺ cells are shown on the right. Bars, 10 μm. (B) Schematic representation of the transgene used to express FOG-1 in the mast cell lineage. Approximately 7 kb of DNA 5′ to the first erythroid exon (1E) of GATA-1 (noncoding) were used. Full-length FOG-1 cDNA was inserted into exon 2 of GATA-1 upstream of the GATA-1 start codon, followed by an SV40 polyA sequence (gray box). Exons are indicated by black boxes. (C) Limiting dilution analysis of bone marrow GMPs from FOG-1–transgenic mice or wild-type littermates. The percentage of wells that failed to generate mast cells was scored, and the number of cells per well leading to a 37% detection failure rate was calculated.