Secondary animals that received BM-cell transplants from responding, primary drug-treated animals demonstrate persistence of high levels of Pro140Lys MGMT/GFP⁺ RBCs and a corrected hematologic phenotype. Secondary transplantation experiments were performed by transplanting the BM cells flushed from the hind limbs of a primary animal into 3 secondary animals. (A) The percentages of Pro140Lys MGMT/GFP⁺ RBCs are shown in animals that received secondary transplants derived from control untreated animals (□; animals 2 and 3) and secondary animals derived from drug-treated animals that demonstrated in vivo selection (▪; animals 6 and 23). * indicates P < .0001 versus both control, untreated data sets. (B) PB Hb concentration in the same secondary recipients as shown in (A). * indicates P < .0084 versus both control, untreated data sets; **P = .038 versus untreated control 2 and P = .055 versus untreated control 3. Analysis was performed at 12 weeks after transplantation and all values represent the mean ± SEM.