Mpl is required for development of JAK2V₆₁₇F⁺ MPNs in marrow transplantation models. Data were collected from WT into WT, JAK2V₆₁₇F⁺ into WT (VF⁺ into WT), Mpl^−/− into Mpl^−/−, JAK2V₆₁₇F⁺Mpl^−/− into Mpl^−/− (VF⁺ into Mpl^−/−), WT into Tpo^−/−, and JAK2V₆₁₇F⁺ into Tpo^−/− mice 16 weeks after transplant. (A) Number of progenitor cells present per 2 × 10⁴ marrow cells as determined by CFU assays in the presence and (B) absence of cytokine. Data are presented as mean ± SEM; n = 2 to 12. (C) CFU-MK per 1 × 10⁵ marrow cells as determined by Megacult assays in the presence of cytokine. Data are presented as mean ± SEM. n = 2 to 12. (D) Representative hematoxylin and eosin-stained sections of bone marrow with (E) megakaryocyte count and (F) size. Scale bar, 50 μm.