(A) Absolute number of KLS cells in long bones of animals treated with ACK2 or control on E14.5 and harvested on P1. * p<0.001, n=5-18/group. (B) Levels of donor peripheral blood CD45 cell chimerism in treated animals after in utero ACK2 treatment and neonatal transplantation, or in utero transplantation alone. * p<0.05 between control and all doses, n≥4/group, * * p<0.05 between 2.5 µg or 5 µg and in utero transplant at 12 weeks, n≥4/group.