Mice homozygous for the R26Fki knock-in allele (FLPeR mice) are viable and fertile, with widespread expression of the FLPe variant of the Saccharomyces cerevisiae FLP1 recombinase gene driven by the Gt(ROSA)26Sor promoter. The FLPe recombinase variant exhibits enhanced thermostability with recombination activity being four-fold and ten-fold that of wild type FLP at 37C and 40C, respectively. The knock-in construct also includes a neomycin resistance gene driven by the mouse phosphoglycerate kinase promoter (PGK neo) and polyadenylation signal sequence. The Gt(ROSA)26Sor promoter drives expression from preimplantation onward and extensive target gene recombination can be achieved in most tissue types, including cells of the developing germ line. When bred with mice containing a frt-flanked sequence of interest, FLP-mediated recombination will result in deletion of the frt-flanked sequence(s) in the offspring. This FLP deleter strain may be useful for generating frt-FLP conditional mutations and serves as a source of FLP1-expressing primary cell lines. The Jackson Laboratory 2024, used with permission.