A
gene activation / inactivation and site-specific integration
system has been developed for mammalian cells. The invention
system is based on the recombination of transfected sequences by FLP, a
recombinase derived from
Saccharomyces. In several
cell lines, FLP has been shown to rapidly and precisely recombine copies of its specific target sequence. For example, a chromosomally integrated, silent β-galactosidase
reporter gene was activated for expression by FLP-mediated removal of intervening sequences to generate clones of marked cells. Alternatively, the reverse reaction can be used to target transfected
DNA to specific chromosomal sites. These results demonstrate that FLP can be used, for example, to mosaically activate or inactivate transgenes for a variety of therapeutic purposes, as well as for analysis of
vertebrate development. The FLP recombination
system of the present invention can be incorporated in transgenic, non-human mammals to achieve site-specific integration of transgenes, to construct
functional genes or to disrupt existing genes.