Fetal X-SCID recipients are more permissive to congenic HSC reconstitution of the T-cell lineage than postnatal and adult recipients. Neonatal (postnatal days 1-4) and young adult (4-6 weeks old) X-SCID mice were transplanted without any conditioning with 200 congenic LMPPs (LSK CD34⁺ FLT3^hi) or 200 HSCs (LSK CD34⁻ FLT3⁻). PB reconstitution analysis of total, B cells, and T cells was performed at 3 to 4, 6, and 16 weeks after transplantation. Shown are results from 4 neonatal (A-C) and 3 adult (D-F) transplantation experiments. Numbers above bars indicate frequencies of transplanted mice with more than 0.1% total reconstitution at each time point. Statistical significance in reconstitution levels between LMPP and HSC transplanted recipients was evaluated, comparing all recipients alive (positive and negative for reconstitution) at analysis time point. ***P < .001; **P < .005; *P < .05. (G,H) Donor contribution of 200 congenic LMPPs (left panel) and 200 HSCs (right panel) to (G) B-cell and (H) T-cell reconstitution analyzed at 16 weeks after transplantation in fetal (), neonatal (), and adult () recipients. Statistical differences comparing reconstitution levels in all transplanted and alive recipients are indicated above bars. ***P < .001; **P < .005; *P < .05.