Increased Fas expression is the molecular basis of the increased differentiation observed in Raf-1-deficient erythroblasts. Fas surface expression (A) and erythroid differentiation (B) are increased by Raf-1 reduction/ablation. Fas surface expression and hemoglobin content were determined at the indicated times during differentiation in erythroid cells of indicated genotypes. (C) The lpr/lpr mutation prevents terminal differentiation of WT, Raf-1^+/-, and Raf-1^-/-erythroblasts. Hemoglobin accumulation is shown as a differentiation parameter. One representative experiment of 4 is shown in the 3 panels.