ERK MAPK activity is not required for myeloid differentiation of hematopoietic progenitor cells. (A-C) CD34⁺ cells were cultured either in the presence of G-CSF to induce neutrophil differentiation or in the absence of 5 μM U0126. (B) After 14 and 17 days of culture, cytospins were prepared and stained with May-Grünwald-Giemsa solution. Data were expressed as either (A) the percentage of differentiated cells or (C) the absolute cell numbers. Data were expressed as the percentage of differentiated neutrophils. (D) Lactoferrin expression was analyzed by FACS to determine neutrophil development. (E) Freshly isolated CD34⁺ progenitor cells were plated in CFU assays, either in the presence or in the absence of 5 μM U0126, and colony formation was analyzed after 12 days. (F) Freshly isolated CD34⁺ progenitor cells were plated in CFU assays, either in the presence or in the absence of 5 μM U0126, and absolute cell numbers of total GM colonies were analyzed after 12 days. Results are presented as means of 4 independent experiments. Error bars represent SEMs.