MEK activity is essential for the survival of hematopoietic progenitors during erythropoiesis. CD34⁺ progenitors were differentiated toward erythrocytes for 18 days either in the presence or in the absence of 5 μM U0126. (A) Expansion was determined by counting the trypan blue–negative cells. (B) After 18 days of culture, cytospins were prepared and stained with May-Grünwald-Giemsa solution. (C) During the 18-day culture period, the percentage of apoptotic cells was determined by annexin V staining. Results are presented as means of 4 independent experiments. Error bars represent SEMs. (D) Protein lysates were prepared from CD34⁺ cells differentiated toward erythrocytes for 8, 11, and 15 days in the presence or absence of 5 μM U0126. Western blot analysis was performed with an antibody against Bcl-xL (top) or β-actin (bottom) as a control for equal loading. Similar results were obtained in 3 independent experiments.