(A) Sequential expression of genes required for ventral mesoderm induction, hemangioblast formation, and hematopoiesis and vasculogenesis during control EB differentiation. (B) Pattern of gene expression observed during differentiation of scl-null EBs showing a failure of expression of hematopoietic-restricted genes. The data for this figure was derived from gene expression in embryoid bodies harvested at 4, 6, and 8 days of differentiation compared with the expression of each gene in undifferentiated ES cells. Expression levels were quantitated on the phosphorimager as described in Materials and Methods. The expression of each gene in undifferentiated ES cells was normalized to the base line and the maximal level of expression given the same arbitrary value for each gene. Gene expression is shown on a logarithmic scale on the Y axis, and the day of differentiation is indicated on the X axis. The points have been linked to form a curve to make the data easier to appreciate. The approximate timing of the major developmental processes of mesoderm induction, hemangioblast formation, and blood cell differentiation are shown in boxes above the curves.