Mpl expression from lineage-specific promoters in lentiviral vectors corrects defective megakaryopoiesis of Mpl^−/− cells in vitro. (A) Lin⁻Mpl^−/− BM cells were transduced with SIN lentiviral vectors expressing Mpl from indicated promoters. After differentiation in medium containing Thpo, stem cell factor, and interleukin-6, MK numbers were determined by flow cytometry as CD41⁺CD42⁺ cells. C57Bl/6 (wt) cells were used as positive control (n = 3, mean ± SEM). ***P < .001 (Student t test, 2-tailed, unpaired). (B) Cytospins of in vitro cultured Mpl^−/− cells (left) and Mpl corrected cells (GPIba.Mpl, right). May-Grün-Wald/Giemsa staining; original magnification, ×100.