Expanded megakaryocytopoiesis in recipients of Mpl^+/+p300^Plt6/Plt6 bone marrow. Recipients of Mpl^+/+p300^Plt6/Plt6 (6/6) and Mpl^+/+p300^+/+ control (+/+) bone marrow exhibited (A) increased numbers of megakaryocytes (megs), shown as number per microscopic field (×600, BM; ×200, spleen), and (B) megakaryocyte colony-forming cells (meg-CFCs), shown as number per 2.5 × 10⁴ bone marrow (BM) or 10⁵ spleen cells. Means plus or minus SD are shown. * P value less than .05 for comparison of data from recipients of Mpl^+/+p300^Plt6/Plt6 marrow with that of Mpl^+/+p300^+/+ marrow recipients. n = 4 mice per genotype.