EGFP expression in hematopoietic cells in recipient mice undergoing secondary transplantation. C57BL/6 mice underwent DII of MGIN retrovirus and transplantation, and then EGFP⁺ BMCs were obtained from these mice after 6 months by FACS sorting and transplanted into secondary irradiated recipient mice as described in “Materials and methods.” Two to 5 months following secondary transplantation of 50 000 EGFP⁺ cells, PB was analyzed for the presence of donor cells in 3 separate mice (A). (B) In vivo gene transfer to PHSCs. Four months following secondary transplantation, PB was analyzed for GFP expression in T cells (Thy 1.2), B cells (B220), and granulocytes (Gr-1) from a representative animal.