Survival, VCNs, and absolute numbers of lymphocytes (total and gene-marked) after conditioning with 200 cGy and 900 cGy of TBI. Tissues were harvested 4 months after recipients received transduced cells. VCN was determined via qPCR. Thymic subpopulations and splenic subpopulations were determined by flow cytometry. The absolute numbers of lymphocytes were determined by multiplying the percentage of cells in a subpopulation by the total number of lymphocytes in the lymphoid tissue. The absolute numbers of gene-marked cells were determined by multiplying the absolute numbers of thymocytes or splenocytes by the average VCN in thymus or spleen, respectively. (A) Survival of recipients. Mice were killed and analyzed at day 120. Total mice in 5 experiments. (B) VCN in tissue cell suspensions and isolated cell populations from the thymus (CD4/8), spleen (CD19), and marrow (CD11b) determined by qPCR for vector sequence. (900+ERT, n = 14; 900−ERT, n = 12; 200+ERT, n = 12; 200−NoERT, n = 8; mean ± SEM). *Significantly greater marking with 900 cGy compared with 200 cGy (P < .05). **Significantly greater marking with ERT compared with without ERT (P < .05). (C) Absolute numbers of thymocyte and splenocyte populations (mean ± SEM). *Significantly greater number of cells with 200+/ERT compared with 900 ± ERT (P < .05). (D) Absolute numbers of gene-marked thymocyte and splenocyte populations (mean ± SEM). *Significantly greater number of marked cells with 900+ERT compared with 200+ERT and 900−NoERT compared with 200−NoERT (P < .05). **Significantly greater number of gene-marked cells with 900+ERT compared with 200+ERT only (P < .05).