Impaired HSC function in Mysm1^tm1a/tm1a mice. (A) Mysm1 gene expression in KLS (cKit⁺ lineage⁻ Sca1⁺) cells of mouse BM, as shown by the activity of a Mysm1 promoter–driven β-galactosidase reporter; data are presented as fold increase in reporter activity in Mysm1^+/tm1a relative to wild-type cells (*P < .05, one-sample t test). (B) The absolute numbers and proportion of KLS cells in the BM of wild-type and Mysm1^tm1a/tm1a mice. (C) Flow cytometric plots of the BM, stained for cKit and Sca1, and gated on the lineage-negative cells, showing a relative expansion of the KLS population in Mysm1^tm1a/tm1a mice. (D) Proportion of long-term stem cells (LT-SCs, Flt3⁻CD34⁻), short-term stem cells (ST-SCs, Flt3⁻CD34⁺), and multipotent progenitors (MPPs, Flt3⁺CD34⁺) within the KLS population. (E) Reduced Flt3 and elevated CD150 expression in Mysm1^tm1a/tm1a KLS cells. (F) Reduced contribution of Mysm1^tm1a/tm1a cells to the myeloid lineage and the primitive KLS population in mixed BM chimeras. CD45.1⁺ wild-type and CD45.2⁺Mysm1^tm1a/tm1a BM, mixed at 1:1 ratio, were reconstituted into lethally irradiated Rag1^−/− recipients. All bars represent means ± SEM. Cell counts per 1 tibia and femur; *P < .05, **P < .01, ***P < .001 using (B,E,F) t test or (D) ANOVA; MFI indicates median fluorescence intensity.