(A) Transplantation (TX) setup. The HSC-enriched fraction (Lin- CD3-) of BM from heterozygous Setd1bFD/+;RC/+ or homozygous Setd1bFD/FD;RC/+ donor mice (CD45.2+) was intravenously injected into lethally irradiated B6.SJL recipients (CD45.1+) (3 × 104 cells per TX). (B) Successfully reconstituted mice (Ctrl n = 8, cKO n = 14) were induced with tamoxifen: cohort 1 at 20 weeks, cohort 2 at 28 weeks after TX (each cohort with Ctrl n = 4, cKO n = 7). (C) FACS analysis of the CD45.1/2 chimerism in peripheral blood (cohort 1). (i) Before induction, high percentages (92–95%) of donor-derived CD45.2+ blood cells were reached in all transplanted mice. Immediately after induction, this ratio declined in cKO transplanted mice, whereas it remained high in controls (p*<0.05, p***<0.001). (ii) The contribution of CD45.2+ cells to CD11b+ Gr-1high granulocytes was comparable between cKO transplanted mice and controls. (D) Blood count analysis, including data from both cohorts, before and after induction (early = 4–6 weeks, mid = 11–12 weeks, late = 21–22 weeks). (i) After an initial decline, cKO transplanted mice remained lymphocytopenic. (ii) Neutrophil granulocytes as well as (iii) red blood cells revealed normal levels except for a slight decrease at late stages. (iv) Platelet counts were highly diminished at all stages (p*<0.05, p***<0.001). (E) Wright-Giemsa stained blood smears of cKO transplanted mice 48 weeks after induction. Both immature forms and nuclear dysplasia of myeloid cells were noted. (F) cKO transplanted mice showed mildly increased (p*=0.044) spleen weights (Ctrl n = 8, cKO n = 12; 22 + 48 weeks after induction). (G) FACS analysis of splenic cell compartments. While CD11b+ Gr-1high and CD11b+ Gr-1low myeloid cells were increased (p**=0.003), CD19+ B cells but not CD3+ T cells were decreased (p**=0.002) in cKO transplanted mice. (H) FACS analysis of HSPC populations in the BM. cKO transplanted mice revealed abnormal HSPC ratios within the LSK compartment. (I) While MPPs were enriched (p*=0.019), LMPPs were almost undetectable (p*=0.011). All graphs depict the mean ± SD (data derived from two independent experiments).