Expanded megakaryocytopoiesis in Mpl^−/−Plt6 mutants. Elevated platelet counts in Plt6 homozygous (6/6) and heterozygous (6/+) mice were accompanied by (A) increased numbers of megakaryocytes (megs), shown as number per microscopic field (×600, BM; ×200, spleen), (B) megakaryocyte colony-forming cells (meg-CFCs), shown as number per 2.5 × 10⁴ bone marrow (BM) or 10⁵ spleen cells, and (C) colony-forming units–spleen (CFU-s's), shown as number per 1.5 × 10⁵ donor bone marrow cells at days 12 or 8 following transplantation. Means plus or minus SD are shown. * P value less than .05 for comparison of data from Mpl^−/−Plt6/Plt6 or Mpl^−/−Plt6/+ mice with that of Mpl^−/−+/+ mice. n = 3 to 7 mice per genotype.