Study design. (A) Retroviral vectors, expressing MDR1, dsRED2, or human truncated CD34 (htCD34). SFFVp (spleen focus forming virus LTR), HaMSV (Harvey murine sarcoma virus LTR), gag (residual retroviral gag sequences), env (residual retroviral envelope sequences), VL30 (residual sequences of virus-like 30 element), Ψ (packaging signal), wPRE (woodchuck hepatitis virus posttranscriptional regulatory element). rSD, retroviral splice donor; rSA, retroviral splice acceptor. (B) Experimental setup; bone marrow cells were transduced and transplanted 2 or 10 days later in lethally irradiated C57BL/6J recipient mice. After a 7-month observation, secondary recipients underwent transplantation. Symptomatic animals were killed for detailed analysis. All other mice were analyzed at the end of the experiment. MOI indicates multiplicity of infection.