Conditional activation of MEK1 does not affect terminal differentiation of neutrophil progenitors. (A-B) CD34⁺ cells were transduced with MEK1:ER*, sorted by FACS from the nontransduced cells, and differentiated to neutrophils in the presence or absence of 4-OHT. After 3 days of culture, cells were washed and resuspended in normal culture medium containing G-CSF. After 14 and 17 days of differentiation, cytospins were prepared, and the morphology of the cells was analyzed by May-Grünwald-Giemsa staining. Data were expressed as either (A) the percentage of differentiated cells or as (B) the absolute cell numbers. Data were expressed as the percentage of differentiated neutrophils. (C) Lactoferrin expression was analyzed by FACS to determine neutrophil development. (D) CD34⁺ cells, transduced with MEK1:ER*, were sorted by FACS from the nontransduced cells and plated in CFU assays, either in the presence or absence of 20 nM 4-OHT, and colony formation was analyzed after 12 days. Results are presented as means of 4 independent experiments. Error bars represent SEMs.