Absence of constitutive activation of STATs. CD34⁺ cells from 4 PMF patients were cultured without growth factor. After 10 days of culture, CD41⁺ cells were collected and incubated without (lanes 3 through 6) or with (lanes 7 through 10) PEG-rhuMGDF (30 ng/mL) for 15 minutes. Nuclear extracts were prepared and the presence of activated STATs tested by EMSA using IRF1-GAS as a probe. In parallel, as internal controls Mpl-L responsive UT7/huc-mpl cells were incubated in the presence of IFN-γ (lane 1) or PEG-rhuMGDF (lane 2) for the same time, and nuclear extracts were prepared and tested for the presence of activated STATs.