Enhancement of engraftment by a nonmyeloablative postnatal “boosting” regimen in recipients of IUHCT. Chimeric SCD, Thal, and B6 WT mice were boosted at (A) 4 weeks or (B) 32 weeks of age following conditioning with one of 3 doses of irradiation (138 cGy, 82 cGy, and 0 cGy). Engraftment levels of control chimeric SCD, Thal, and WT mice that did not receive a “booster” transplant are included in (B) for comparison. Error bars in (A) and (B) are expressed as SEM. Levels of chimerism within each group (WT, Thal, and SCD) are statistically different (P < .05) following boosting compared with the nonboosted control at that time point, with the exception of the points marked *. Levels of chimerism were statistically different between each irradiation dose within each group (WT, Thal, and SCD) at the indicated time point with the exception of the points marked # (WT, 82 cGy vs 138 cGy), $ (Thal, 82 cGy vs 138 cGy), and & (SCD, 4 weeks, 0 cGy vs 82 cGy; 32 weeks, 82 cGy vs 138 cGy). (C) PB was assessed for multilineage donor-cell engraftment at 28 weeks of age in naïve SJL/J donor mice and chimeric SCD, Thal, and WT mice following postnatal “booster” transplants with 138 cGy conditioning at 4 weeks of age.