HSC behavior in competitive transplantation experiments. (A) Various ratios of test and PeP3^b (WT) marrow cells (1:1, 2:1, 4:1, 8:1) were transplanted into irradiated recipient ROSA26 (WT) or tpo^−/− mice and their contributions to blood granulocytes, marrow granulocytes, and marrow progenitor cells were assessed 3 months later. The expected percentages of test cell engraftment (50%, 66%, 80%, and 88%, respectively) were only seen when C57BL/6J and PeP3^b cells were transplanted into tpo^−/− (i) (or into WT; data not shown) recipients. c-mpl^−/− HSCs are poor competitors when transplanted into WT (thrombopoietin-producing) recipients (iii) yet compete appropriately at marrow cell dose ratios of 4:1 and 8:1 when transplanted into tpo^−/− recipients (ii). See “Results” for details. (B) PeP3^b cells outcompete c-mpl^−/− cells in a WT host. Data from panel A are plotted as percentage PeP3^b cells transplanted versus percentage PeP3^b cells engrafting. Error bars indicate the mean ± standard deviation (SD) of experiments using 3 to 5 mice.