I nfusion of transduced hematopoietic stem cells into nonmyeloablated hosts results in ineffective in vivo levels of transduced cells. To increase the proportion of transduced cells in vivo, selection based on P140K O⁶-methylguanine-DNA-methyltransferase (MGMT[P140K]) gene transduction and O⁶-benzylguanine/1,3-bis(2-chloroethyl)-1-nitrosourea (BG/BCNU) treatment has been devised. In this study, we transduced human NOD/SCID repopulating cells (SRCs) with MGMT(P140K) using a lentiviral vector and infused them into BG/BCNU–conditioned NOD/SCID mice before rounds of BG/BCNU treatment as a model for in vivo selection. Engraftment was not observed until the second round of BG/BCNU treatment, at which time human cells emerged to compose up to 20% of the bone marrow. Furthermore, 99% of human CFCs derived from NOD/SCID mice were positive for provirus as measured by PCR, compared with 35% before transplant and 11% in untreated irradiation-preconditioned mice, demonstrating selection. Bone marrow showed BG-resistant O⁶-alkylguanine-DNA-alkyltransferase (AGT) activity, and CFUs were stained intensely for AGT protein, indicating high transgene expression. Real-time PCR estimates of the number of proviral insertions in individual CFUs ranged from 3 to 22. Selection resulted in expansion of one or more SRC clones containing similar numbers of proviral copies per mouse. To our knowledge, these results provide the first evidence of potent in vivo selection of MGMT(P140K) lentivirus–transduced human SRCs following BG/BCNU treatment.