Colony assays for the quantification of hematopoietic progenitors. Bone marrow and spleen cells of transgenic VavCre;FF1 (n = 4) and MxCre;FF1 mice (n = 4) and controls (n = 6) were seeded in methylcellulose containing mouse interleukin-3 (IL-3), human IL-6 and IL-9, mouse stem cell factor, and human erythropoietin (A), or in collagen media containing thrombopoietin, mouse IL-3 and human IL-6 (B). Colonies were enumerated at day 8. Prior to counting, collagen cultures were fixed and stained for acetyl cholinesterase activity to visualize megakaryocytes. CFU-GEMM, colony forming unit-granulocyte, erythroid, macrophage, megakaryocyte; CFU-GM, sum of colony forming unit-granulocyte, colony forming unit-macrophage and colony forming unit-granulocyte, macrophage; BFU-E, burst forming unit-erythroid. Spleens of double transgenic mice showed marked increase in hematopoietic progenitors. For statistical analysis, we performed pairwise Mann-Whitney tests. Asterisks indicate statistically significant differences (P < .05; in panel A for all colony types)