FOG-1 expression pattern during early hematopoiesis. The expression pattern of FOG-1 and GATA factors was visualized by WISH during zebrafish embryonic development (A-O). The maternal expression of FOG-1 (A-B) and GATA-2 (K-L) transcripts is initially noted at the 2-cell and sphere stages. The zygotic expression pattern of FOG-1 in the blood island is coincident with the expression of GATA-1 and GATA-2 in the LPM (C,H,M) and the ICM (D-E,I-J,N-O). FOG-1 is highly expressed in the ICM and neural tissues of a wild-type embryo at 24 hpf (P). Mutants involved in defective formation of hematopoietic (cloche, clo; Q) or erythroid progenitors (vlad tepes, vlt; R) are deficient in the expression of FOG-1 in the ICM (black arrow), whereas expression in the heart is preserved (white arrow, inset magnification). The expression of FOG-1 in the ICM is normal in a mutant with a defect in late erythroid maturation (frascati, frs; S).