Northern blot analysis of cells transduced by HIV-based (LT) and MLV-based (RT) promoter traps and control MLV-based (R) and HIV-based (L) vectors. Analysis was performed on 293T cells transduced with the indicated vectors, before or after puromycin selection, and in representative puromycin-resistant clones. Cells were transduced with matched vector doses yielding multicopy integration (average of 10) per cell in the unselected cells (LT and RT), and average single-copy integration in the selected populations (LT/RT selected), clones (LT/RT cl), and in cells transduced by control vectors (L, R). The white arrow indicates the LTR-driven antisense transcript of RT. Note that this transcript is expressed to low extent because of the SIN modification and is better detected in cells containing high numbers of vector integrants. The LT and RT lanes are shown enlarged on the right and after a shorter exposure time for the RT lane to better compare the hybridization pattern.