Mpl is essential for the development of JAK2V₆₁₇F⁺ MPNs. Data were collected from 16-week-old WT, JAK2V₆₁₇F⁺, Mpl^+/−, JAK2V₆₁₇F⁺Mpl^+/−, Mpl^−/−, JAK2V₆₁₇F⁺Mpl^−/−, Tpo^−/−, and JAK2V₆₁₇F⁺Tpo^−/− mice. (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 = 3 to 5. (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 = 3 to 5. (D) Representative hematoxylin and eosin-stained sections of bone marrow with (E) megakaryocyte count and (F) size. Scale bar, 50 μm.