Phosphorylation of GATA-1 is required for fetal liver erythroid cell differentiation. (A) TER 119^- fetal liver progenitor cells were transduced with an empty retroviral vector (MIG) or vectors containing the indicated inserts and cultured in the presence or absence of Epo; 36 hours later live GFP⁺ cells were examined for expression of red cell differentiation markers, percentages of CD71⁺TER 119⁺ differentiated cells (top right quadrant) in the absence or presence of Epo are shown. Representative of at least 3 independent experiments. (B) Representative field of Wright-Giemsa staining of live GFP⁺ cells (× 1000) and of differential count: the percentage of highly mature erythroblasts (normoblasts shown by arrows) formed in Epo-stimulated cultures in indicated GFP⁺-transduced cell populations is shown. Images were captured with a Nikon E600 microscope (Garden City, NJ) with a 100×/0.3 numeric aperture oil immersion lens and an RT slider SPOT 2.3.1 camera (Diagnostic Instuments, Sterling Heights, MI) using SPOT advanced software (version 3.5.9).