PhiC31 integrase facilitates genetic approaches combining multiple recombinases

Homologous and site-specific DNA recombination has revolutionized genetic engineering. The reliability of recombinases such as Cre and FLP has allowed scientists to design complex strategies to study gene function in mammals. However, the retention of recombination sites in the genome limits the use of Cre and FLP recombinases in subsequent modifications. Access to additional recombinases in the ES cell toolbox would enormously widen the number of possibilities to manipulate the genome. In the method presented here, we combine the use of PhiC31, a site-specific integrase, with FLP to obtain site-specific insertion and replacement in pre-inserted docking sites in the genome of mouse ES cells. This method allows for the integration of any sequence of interest in a pre-defined locus, leaving Cre recombinase available for downstream applications. The selection strategy is based on a silent selection marker activated by a plasmid-delivered promoter, making the integration system highly reliable and reducing the need for extensive molecular screens. This article describes how to create " dockable" mouse embryonic stem (ES) cell lines, integrate incoming vectors, and analyze the resulting clones. Current applications of this technology are also discussed.