βH1-globin expression is associated with yolk sac–derived definitive hematopoietic progenitors. (A) E9.5 yolk sac progenitors (c-kit^hi, CD41^hi) cultured in a 2-step erythroid differentiation assay express βH1-globin as they mature and switch to predominantly β1-globin expression. SEM of 3 experiments is indicated by error bars. (B) Globin expression of cells derived from individual BFU-E–derived cultures expressing βH1-globin are only found in the yolk sac and early fetal liver. Pale red BFU-E colonies at day 2 to 4 methylcellulose cultures were analyzed individually by qPCR for expression of εy-, βH1-, and β1-globin. Proportions of colonies with βH1-globin expression over the total colonies analyzed are shown.