Increased efficacy of in utero ACK2 treatment compared with neonatal ACK2 treatment. (A) CD45.2 fetuses were injected with ACK2 in utero and transplanted at P1 with FLMCs from CD45.1/CD45.2 mice. They received a second ACK2 treatment at 2 weeks followed by transplantation of FLMC from CD45.1^+/+ mice at 3 weeks of age. Control mice underwent transplantation without ACK2 treatment. (B) Total number of BM KLS cells within long bones 7 days after ACK2 treatment (25 µg/g) at 2 weeks of age. N = 5 per group; **P < .01 by Student t test. (C) Serum concentrations of ACK2 after ACK2 treatment at age 2 weeks. N = 6. (D) Gating strategy to detect host (CD45.2), first donor (CD45.1/CD45.2), and second donor (CD45.1^+/+) leukocytes. (E) Levels of chimerism among mice that received multiple transplantations. N ≥ 5 per group. *P < .05 comparing peripheral blood CD45 chimerism from the first transplant between control and treated mice by Student t test. There were no differences in the levels of chimerism resulting from the second transplantation between control and treated mice.