recombinase did not work in mammals. The FLP protein was thermolabile (denatured at elevated temperatures) and therefore was not useful in the mammalian model due to elevated body temperatures of these model systems. However, due to patents and restrictions on the use of Cre-Lox recombination, great interest was taken to produce a more thermostable FLP-FRT cassette. Some of the first results were produced by Buchholz et al. (1997) by utilizing cycling mutagenesis in Escherichia coli . In their research, the authors transfected E. coli cells with two plasmids: one coding for randomly mutated FLP proteins downstream of an arabinose promoter and another containing a lacZ gene promoter within a FRT cassette. The E. coli were grown on arabinose plates at 37 °C and 40 °C, and if recombination occurred, the lacZ expression would be attenuated, and the colonies would appear white. White colonies were selected from each generation and grown on a new arabinose plates at same previous temperatures for eight generations. After recombination was confirmed by western-blotting and the mutated FLP genes were sequenced, this eighth generation FLP protein (FLPe) was transfected into mammalian cell culture, and recombination in mammalian cells was confirmed. This variant of FLP only has 4 amino acid substitutions: P2S, L33S, Y108N, and S294P. Generation of genetic mosaics Genetic mosaicism occurs within an organism when similar cell types express different phenotypes due to dissimilar genotypes at specific loci. Simply put, this occurs when one organism contains different genotypes, which is usually rare in nature. However, this can be easily (and problematically) produced using FLP-FRT recombination. If two different FRT sites are present within a cell, and FLP is present in appropriate concentrations, the FRT cassette will continue to be excised and inserted between the two FRT sites. This process will continue until the FLP proteins fall below the required concentrations resulting in cells within an organism possessing different genotypes. This has been seen from fruit flies to mice and is indiscriminate against specific chromosomes (somatic and sex) or cell types (somatic and germline). Determination of cell lineages Before the publication of Dymecki et al. (1998), Cre recombinase had been used for cell-fate mapping of neuronal progenitors in mice using the En2 promoter. Thus, the authors of Dymecki et al. (1998) theorized that FLP recombinase could be utilized in a similar fashion with a similar efficacy as Cre recombinase in mice. The authors created a two