Transduction of 5FU-treated BM with the MBS vector. (A) Structure of the MSCV-huBtk-SAR (MBS) retroviral vector. The murine stem cell virus (MSCV) promoter drives expression of human Btk. The human interferon-β scaffold attachment region (SAR) was included to facilitate sustained expression of the Btk transgene in differentiated cells. (B) Btk expression in mock-transduced BM from WT and BtkTec^–/– mice, and in MBS-transduced BM from BtkTec^–/– mice. BM was analyzed immediately prior to transplantation into BtkTec^–/– recipients (using intracellular FACS analysis). Btk expression in BM progenitors was between 6% and 11% in experiments 1 to 3.