Intraperitoneally engraftment of newborn RAG2^-/- γc^-/- mice at 1 day, 1 week, or 2 weeks of age, with CD34⁺ fetal liver (FL) cells. Three groups each of 4 newborn RAG2^-/- γc^-/- mice—1 day (▴), 1 week (○), or 2 weeks (×) of age—were intraperitoneally grafted with 0.5 × 10⁶ MACS-selected CD34⁺ FL cells. At 4, 8, and 12 weeks after engraftment, all mice were bled, and the blood was subjected to fluorescence-activated cell sorter (FACS) analysis to determine the percentage of CD45⁺ human cells.