As conditional genetic strategies advance, the need for multiple site-specific recombinase systems has emerged. To meet this need in part, we have targeted the constitutive ROSA26 locus to create a mouse strain with generalized expression of the enhanced version of the site-specific recombinase FLP (FLPe). This strain is designated FLPeR ("flipper"). Using this strain, extensive target gene recombination can be achieved in most tissue types, including cells of the developing germ line. FLPeR mice therefore serve two important functions: as a source of many different FLPe-expressing primary cell lines and as a deleter strain. Moreover, because the FLPeR mouse is a 129-derived strain, a 129 genetic background can be preserved when crossed to most ES cell-derived mice. This enables conditional genetic alterations to be maintained on a standard background, a feature important for obtaining reproducible results and genetically defined controls.