Increased number of platelets, megakaryocytes, and hematopoietic progenitors in Yall;Mpl^−/− mice. (A) Platelet counts in peripheral blood from wild-type (wt), and nontransgenic (−), or transgenic (Yall) Mpl^−/−mice. (B) Frequency of megakaryocytes in the bone marrow of mice with the same genotypes as in panel A, determined by flow cytometry. (C) Ploidy of megakaryocytes with the indicated genotypes, determined by flow cytometry. (D) CFU-MK assays showing an increased number of megakaryocyte (MK) colonies in Yall;Mpl^−/− mice. (E) Hematopoietic colony assays (CFUs/burst-forming units [BFUs]) in methylcellulose indicate a general expansion of progenitors in the bone marrow of Yall;Mpl^−/− mice. Error bars in panels A through D represent SD.