Ezh1 was essential for the maintenance of Ezh2 insufficient hematopoiesis. (A) The failure of Ezh1^−/−Ezh2^Δ/Δ hematopoietic cells to maintain hematopoiesis. BM cells from Cre-ERT and Cre-ERT;Ezh1^−/−Ezh2^fl/fl mice were transplanted into lethally irradiated recipient mice without rescue BM cells, and Ezh2 was then deleted by injecting tamoxifen at 6 weeks posttransplantation. Following tamoxifen injection, the recipient mice were fed with a tamoxifen diet (400 mg of tamoxifen citrate per kg diet) for 10 days. WBC counts, the chimerism of CD45.2⁺ donor-derived cells, including Mac-1⁺ myeloid cells, B220⁺ B cells, and CD4⁺ or CD8⁺ T cells, in the PB from WT (n = 5) and Ezh1^−/−Ezh2^Δzh (double knockout [DKO]) mice (n = 5) are shown as the mean ± SD. (B-C) Competitive repopulating assays. BM cells (CD45.2) from Cre-ERT, Ezh1^−/−, Cre-ERT;Ezh2^fl/fl, and Cre-ERT;Ezh1^−/−Ezh2^fl/fl mice were transplanted into lethally irradiated recipient mice (CD45.1) with twice more competitor BM cells (CD45.1), and Ezh2 was then deleted by injecting tamoxifen at 8 weeks posttransplantation. The chimerism of donor-derived CD45.2⁺ cells, Mac-1⁺ myeloid cells, B220⁺ B cells, and CD4⁺ or CD8⁺ T cells in the PB is shown as percentage of chimerism values prior to the treatment with tamoxifen in panel B. Donor chimerism in BM LSK cells at 12 weeks posttransplantation is shown in panel C. Data are plotted as dots and the mean values are indicated as bars (n = 3-4, each). (D) A quantitative reverse transcription PCR (RT-PCR) analysis of CDK inhibitors (p15^Ink4b, p16^Ink4a and p19^Arf) in WT, Ezh2^Δ/Δ, and Ezh1^−/−Ezh2^Δ/Δ LSK and LK cells 2 weeks after the deletion of Ezh2. mRNA levels were normalized to Hprt1 expression and relative expression levels are shown as the mean ± SD for triplicate analyses. The cells whose expression was arbitrarily set to 1 are indicated as “1”. Statistical significance of difference was measured by unpaired 2-tailed Student t test. *P < .05, **P < .01, ***P < .001.