FIG. 2.

Assay for shielding of the PBS silencer by insulators. A retroviral vector with a B2 PBS and an internal AdMLP driving neo expression is transfected into transient-packaging cells. Virus particles contain a B2-Pro mismatch at the PBS region in the RNA genome, resulting in both Pro PBS- and B2 PBS-containing proviruses in the target cell population. Due to restriction of the Pro PBS vector in EC cells, only the B2 PBS vector will be present after selection for neo expression (right), while a DNA element (X) capable of shielding the internal promoter from the silencer gives rise to a cell population of both Pro and B2 proviruses (left).