Comparison of hematopoietic progenitors in the marrows of WT, mpl^−/−, and Tg(mpl) mice. (A) BFU-E, CFU-GM, CFU-GEMM, and CFU-Mk progenitors were grown in methylcellulose and collagen–based media and enumerated; + indicates no CFU-GEMM colonies grew from mpl^−/− marrow; n = 3 mice from each strain. *P = .006 versus mpl^−/−; **P ≤ .013 for WT versus mpl^−/− and for Tg(mpl) versus mpl^−/−; ***P ≤ .014 for WT versus mpl^−/− and for Tg(mpl) versus mpl^−/−. (B) Size of CFU-Mk colonies. The CFU-Mk colonies (total counts shown in panel A) were further characterized by size. There were significantly fewer large (> 20-cell) colonies in Tg(mpl) than WT marrow (P < .005). Means plus or minus SE are shown.