Excision of an integrated provirus by the action of FLP recombinase.

Retroviral vectors can be used to insert a single, intact copy of a transgene into a chromosome. If the duplication of the LTR (long-terminal repeat) that naturally occurs during reverse transcription of the retroviral genome is exploited to introduce two equally oriented FLP recognition target (FRT) sites, a substrate for FLP recombinase is created. A pulse of FLP recombinase activity can then be applied to excise the intervening sequences with the retention of a single LTR. This procedure is of potential use for manipulating an integration site after a period of expression enabling a variety of critical controls. We describe the properties of such a retroviral vector containing a dicistronic expression cassette with a reporter gene in the first and a positive/negative selection marker in the second cistron. This vector permits the selection and control of each step during the sequence of genomic manipulations enabled by site-specific recombination events.