GATA-1 requires the BMP pathway to cooperate with bFGF to induce erythroid cells. Embryos were coinjected with (A) β-gal RNA (1 ng) and pcDNA3–BMP-4 (200 pg); (B) ▵mTFRII RNA (1 ng) and pcDNA3–BMP-4 (200 pg); (C) β-gal RNA (1 ng) and pcDNA3–GATA-1 RNA (200 pg); (D) ▵mTFRII RNA (1 ng) and pcDNA3–GATA-1 RNA (200 pg); (E) β-gal RNA (1 ng) and control vector (200 pg); and (F) ▵mTFRII RNA (1 ng) and control vector (200 pg). Animal caps were explanted, cultured with bFGF (20 ng/mL) until sibling embryos were at stage 35, and analyzed for o-dianisidine–positive cells (Fig 1A). The panels are photographed at 20× original magnification. (G) The number ofo-dianisidine–positive cells was counted in two cytospins for each experimental condition in a single experiment. The number ofo-dianisidine cells per cap was determined as described in Fig4.