Mechanism of vector insertional gene activation. A genomic integration site of a MLV-based retroviral vector in a target cell is depicted. With this MLV vector design, the enhancer and promoter within the U3 region (blue rectangle) of the long terminal repeat (LTR; white rectangles) drive transcription of the transgene (indicated by the parallel arrow arising from the blue rectangle). At top is shown a vector integration near Gene X. The enhancer elements located in the U3 region (blue rectangle) of the vector can interact with the regulatory elements upstream of Gene X to increase the basal level of transcription to inappropriately high levels, potentially altering the growth of the cell. Two alternatives to eliminate the use of the powerful enhancer in the U3 include 1) middle panel: use of a self-inactivating (SIN) MLV-based vector in which the U3 region has been deleted (noted by X) and which utilizes an internal cellular promoter to drive transgene expression (parallel arrow) and 2) bottom panel: use of a SIN lentiviral vector in which the U3 (yellow rectangle) has also been eliminated and, like the SIN MLV vector, uses an internal cellular promoter to drive transgene expression.