Meis1 Deletion in LT-HSCs results in apoptosis and loss of quiescence. (A) Left panel: Representative flow cytometry profile of LT-HSCs (Lin⁻Sca-1⁺Kit⁺Flk2⁻CD34⁻) of bone marrow (BM) cells are shown for control Meis1^+/+ and mutant Meis1^−/− mice. Numbers in the FACS plots indicate percentages among total BM cells. Right panel: Quantification of LT-HSCs demonstrates significantly higher number of HSCs in Meis1^−/− BM (n = 6). (B) The in vitro methylcellulose colony formation assay was performed at the time of sacrifice after tamoxifen injections with BM cells of control and Meis1^−/− mice. CFU-GEMM colonies representing most undifferentiated progenitors type of colonies derived from Meis1^+/+ and Meis1^−/− BM cells demonstrates decreased percentage of CFU-GEMM. Quantification of BFU-E and CFU-GM colonies derived from Meis1^−/− cells shows no differences (n = 3). (C) Left panel: Representative FACS analysis of Pyronin Y/Hoecst staining on LT-HSCs (Lin⁻Sca-1⁺Kit⁺CD150⁺CD48⁻) of Meis1^+/+ and Mesi1^−/− mice. Numbers in the FACS plots indicate percentages among LT-HSCs. Right panel, The quantification of G₀, G₁, or S/G₂/M phase in Meis1^+/+ and Meis1^−/− LT-HSCs (n = 6). (D) Quantification of apoptosis in Meis1^+/+ and Meis1^−/− LT-HSCs (n = 3). (E) Quantification of LT-HSCs in peripheral blood (PB) of Meis1^+/+ and Meis1^−/− mice (n = 6); *P < .05, **P < .01.