Plasma TPO levels and platelet Mpl expression. (A) TPO levels determined by ELISA on peripheral blood plasma from WT, mpl^−/−, and Tg(mpl) mice; n = 3 to 4 mice for each strain. *P ≤ .003, Tg(mpl) versus WT and Tg(mpl) versus mpl^−/−. Means plus or minus SE are shown. (B) Expression of Mpl on platelet lysates analyzed by Western blot probed with Mpl antiserum (top panel). The blot was stripped and reprobed with anti–pan-actin antibody to ensure equal amount of protein in each lane (bottom panel). Levels of Mpl in Tg(mpl) platelets are 9-fold lower than that in WT.