Three alleles of Aml1/Runx1 are not essential for the development of MPD. (A) Hematoxylin and eosin staining of bone marrow and spleen (top 2 panels) and bone marrow reticulin staining (bottom panels) demonstrate that Ts65Dn mice with 3 or 2 alleles of Runx1 (Ts65Dn/Runx1^+/+/+ and Ts65Dn/Runx1^+/+/−, respectively) develop a similar phenotype. (B) In vitro colony-forming assays demonstrate that restoring disomy at the Runx1 locus does not abrogate extramedullary hematopoiesis in Ts65Dn. Runx1 trisomy may contribute to the extent of extramedullary megakaryopoiesis. Error bars represent SE.