Human CD45⁺ cell chimerism in the blood of newborn-engrafted mice at 12 weeks of age. Nonirradiated NRG and NRG-Tg(hu-mSCF) mice and 400-cGy NRG and NRG-Tg(hu-mSCF) mice were engrafted as newborns with human HSCs as described in “Engraftment of mice with human HSCs.” Mice were analyzed by flow cytometry for the percentage of human CD45⁺ cells in the blood at 12 weeks of age. Each symbol represents an individual animal. No significant differences were observed among the nonirradiation groups or among the irradiated groups. Engraftment in either of the nonirradiated groups was significantly different from the engraftment observed in either of the irradiated groups (P < .001).