Mpl gene therapy regenerates HSCs in primary and secondary Mpl^−/− recipient mice. Mpl^−/− mice were transplanted with Lin⁻Mpl^−/− BM cells transduced with SIN lentiviral vectors expressing Mpl from lineage-specific promoters as indicated, or eGFP from the PGK promoter (GFP). Lin⁻ BM from C57Bl/6 was transplanted as positive control (wt). (A) Representative flow cytometry plots of LSK cells from primary mice shown in panels B and C. Percentages of LSK cells indicated. (B) Percentage of LSK cells as determined by flow cytometry in primary and secondary Mpl^−/− recipients on final analysis. Each primary recipient was transplanted into 2 secondary Mpl^−/− mice with 5 × 10⁶ BM cells (median ± SD). *P < .05 (Mann-Whitney test). ***P < .001 (Mann-Whitney test). (C) Percentage of LK cells as determined by flow cytometry in primary recipients on final analysis (median ± SD). *P < .05, **P < .01 (Mann-Whitney test).