Myb-deficient HSCs are compromised in their function. (A) Bone marrow absolute counts of KSL and LT-HSC (KSLCD48-CD150+) from 2- (n = 8) and 12-month-old (n = 8) WT and Myb^+/− mice. KSL^2mP = .0009, KSL^12mP = .053, LT-HSC^2mP = .013, LT-HSC^12mP = .005. (B and D) Sorted KSL cells from 2- (n = 9) and 12-month-old (n = 9) WT or Myb^+/− donors (carrying vWF-tdTomato transgene) plus reference bone marrow were transplanted into lethally irradiated recipients (900 Gy). Reference:donor cell ratios were calculated using flow cytometry. P^2m = 0.040, 0.038 and P^12m = 0.030, 0.006, 0.055. (C) Analysis of donor cell percentage in the periphery of recipient mice (n = 3) 4 months after transplant receiving 2-month-old KSL. Antibody staining gated on CD45.2 donor cells: myeloid (CD11b⁺), T-lymphoid (CD4⁺CD8a⁺), B-lymphoid (B220⁺), and platelet (vWF⁺). Myeloid P = .001, B-lymphoid P = .011.