Expansion of hematopoietic stem and progenitor cells in double-transgenic animals. (A) Bone marrow-derived hematopoietic stem cells (HSC), common myeloid progenitors (CMP), granulocyte-macrophage progenitors (GMP), and megakaryocyte-erythrocyte progenitors (MEP) were analyzed after lineage depletion using multicolor FACS, as described in “Materials and methods.” Left panel and right panel show a representative analysis for an SCLtTA single-transgenic and an SCLtTA/BCR-ABL double-transgenic mouse, respectively; both mice were killed after 21 days of tetracycline removal. (upper panel) Percentage of Lin^-c-kit⁺Sca-1⁺ HSC. (lower panel) Percentage of Lin^-c-kit⁺Sca-1^- GMP, CMP, and MEP populations in the bone marrow. (B) tTA mRNA expression in multicolor FACS-sorted hematopoietic stem cells (HSC), common myeloid progenitors (CMP), common lymphoid progenitors (CLP), granulocyte-macrophage progenitors (GMP), and megakaryocyte-erythrocyte progenitors (MEP). DNase-treated mRNA was reverse-transcribed into cDNA, as detailed in “Materials and methods,” and was subjected to 35 cycles of RT-PCR using primers specific for tTA or murine GAPDH. Sample labels indicate the following: 1, Tet-O-cre mouse; 2, SCLtTA single-transgenic mouse; 3 to 5, SCLtTA/BCR-ABL double-transgenic mice. The lymph node of an SCLtTA/BCR-ABL double-transgenic mouse (W107) served as a positive control, and the spleen of an MMTV/BCR-ABL double-transgenic mouse (W202) was used as a negative control.