(A) Bone marrow cellularities of scurfy mice and their littermate controls at days 7, 14, 21, and 28 after birth. Data shown are mean ± SD (n = 4). The absolute number of bone marrow cells in scurfy and WT mice (left) and those after normalization against body weight (right) are shown. (B) HSC frequency (left) and numbers (right) in scurfy mice. Data shown are the percentage of Flk2^–lin^–Sca-1⁺c-kit⁺CD34^–CD150⁺CD48^– cells in bone marrow of scurfy mice and their littermate controls at days 7, 21, and 28 (mean ± SD). Each time point involves 3–5 mice per group. (C) Hyperproliferation of HSCs in day 21 scurfy (sf) mice. BrdU was labeled in vivo for 24 hours and LSK cells and HSCs were stained with BrdU antibodies. Representative histograms of BrdU staining in gated LSK cell and HSC populations and the percentage of BrdU⁺ population are shown. Numbers indicate the percentage of BrdU⁺ cells. WBM, whole bone marrow cells. Data shown are mean ± SD (n = 4). (D) Diagram of competitive bone marrow transplantation (BMT). At days 7, 21, and 28, 5 × 10⁵ bone marrow cells from scurfy mice or those from their littermate controls were mixed with equal number of recipient-type bone marrow cells and transplanted into lethally irradiated CD45.1 C57BL/6 recipients. (E) Representative profiles of recipient peripheral blood from 28-day-old donors, evaluated at 12 weeks after reconstitution. Numbers indicate the percentage of donor-derived cells (CD45.2⁺, right bottom quadrants) or recipient-derived cells (CD45.1⁺, left top quadrants) in peripheral blood of recipient mice. (F) Reconstitution ratios in the recipient peripheral blood by the donor cells were monitored at 4 and 12 weeks after transplant. (G) Defective reconstitution in both myeloid (M) (CD11b⁺) and lymphoid lineages (B220⁺ for B cells and CD3⁺ for T cells). The bone marrow used are from 28-day-old mice. Data shown in F and G are mean ± SD (n = 10) from 2 independent experiments, involving 1 donor and 5 recipients per group. *P < 0.05; **P < 0.01; ***P < 0.001.