Donor chimerism after neonatal BMT and cytoreduction. (A) Chimerism was determined in tissues from ADA-deficient mice (no cytoreduction, n = 3; busulfan, n = 6; 100 cGy, n = 7; 200 cGy, n = 2; and 400 cGy, n = 1) after 240 days as described in Figure 5. Dose-response of chimerism with increasing TBI dosage is significant in thymus (P < .001), spleen (P < .05), and marrow (P < .001). (B) Chimerism was determined in cells from thymus (CD4⁺ and CD8⁺ T cells), spleen (CD19⁺ B cells), and bone marrow (CD11b⁺ myeloid cells) isolated from ADA-deficient mice (no cytoreduction, n = 5; busulfan, n = 6; 100 cGy, n = 9; 200 cGy, n = 2; and 400 cGy, n = 1) as described in Figure 4B. γC gene knockout mice (Ly5.1) were treated by neonatal infusion of normal congenic bone marrow (Ly5.2, γC^+/+) and killed after 200 days (n = 4). Cell populations were isolated and analyzed as described in Figure 4B. Statistical analysis for ADA^−/− mice only: Dose-response is significant in CD4⁺ cells (P < .001), CD19⁺ cells (P < .001), and CD11b⁺ cells (P < .05).