Mpl is necessary for primitive HSC overexpansion observed in JAK2V₆₁₇F⁺ MPNs. (A) Bone marrow cells harvested from 16-week-old congenital or (B) transplant mice 16-weeks after transplant were analyzed for number of E-SLAM⁺ HSCs by flow cytometry. Data are presented as mean ± SEM; n = 2 to 9 mice per group.