FLP::EBD encodes a ligand-inducible recombinase that is composed of the FLP recombinase encoded by the Saccharomyces cerevisiae 2μ plasmid fused to the estrogen-binding domain (EBD) from the Homo sapiens estrogen receptor gene. The presence of the estrogen-binding domain results in the level of recombinase activity being regulated by estrogen, with low levels of activity in the absence of the hormone (PMID:9178754). Upon activation, FLP::EBD mediates recombination between a pair of specific target sites, with compatible target sites corresponding to the wild-type FRT site and some of its derivatives. The minimal FRT target site is composed of two 13bp inverted repeats flanking an 8bp asymmetric spacer sequence (PMID:3879971). Recombination occurs between a pair of target sites oriented in the same direction; the 13bp repeats each act as binding sites for the recombinase (PMID:3047402), while the asymmetric 8bp spacer is the site of DNA strand exchange and determines the orientation of the target site (PMID:3711092). In addition to being compatible with the wild-type FRT site, FLP::EBD is expected to be compatible with any engineered FRT variant that contains a mutation only in the spacer region. However, FLP::EBD may be incompatible or show reduced recombination efficiency with FRT derivatives that contain mutations in their inverted repeats, since these mutations can affect the ability of the recombinase to bind the target site.