FOG-1 is required for myelopoiesis in developing zebrafish. Uninjected control embryos (A,D,G), FOG-1 MO-injected embryos (B,E,H), vlt mutants (J), and vlt mutants injected with FOG-1 MO (K). Lateral views of embryos fixed at 22 and 24 hpf were processed by single-labeled WISH to reveal the expression of the myeloid markers, PU.1 (A-B) and mpo (D-E), respectively. Lateral views of embryos fixed at 48 hpf were processed by double-labeled WISH to reveal the expression of the myeloid-specific marker mpo (purple) and the erythroid-specific marker band3 (red; G-H,J-K). Black arrow indicates myeloid cells; white arrow, erythroid cells. Higher magnification views of the ICM (square bracket, D-E) and the ducts of Cuvier on the yolk (Ji-Ki). Loss of FOG-1 function results in an increased expression of myeloid-specific markers, PU.1/mpo (A-F) and decreased expression of the erythroid-specific marker, band3 (G-H). Mutation in the GATA-1 gene causes an increase in the number of mpo⁺ cells in the vlt mutants compared with wild-type embryos. Injection of FOG-1MO in vlt further expands mpo⁺ cells compared with vlt control embryos (compare J with K). Genotyping of injected embryos (FOG-1 MO) with increased myeloid markers was performed to verify their vlt (−/−) genotype (L). Bars represent quantification of the normalized phenotypes (mean ± SD) with the numbers of embryos analyzed in each category indicated above the bars (C,F,I,M). White, blue, and black bars represent embryos with normal, increased, and reduced expression for myeloid markers, respectively. Results were derived from 3 independent experiments. The statistical significance, marked by lines for each paired conditions, was analyzed using analysis of variance: *P < .001; **P < .05.