The FRT entry in FlyBase represents the wild-type target site for the FLP recombinase encoded by the Saccharomyces cerevisiae 2μ plasmid. It is expected to be compatible with any engineered derivative of FLP in which the target site specificity of the recombinase has not been altered. The minimal 34bp FRT site (5'-GAAGTTCCTATTCtctagaaaGTATAGGAACTTC-3') 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). Sequence homology between the spacer sequence of the two target sites is required for efficient recombination to occur (PMID:3711092), and thus the wild-type FRT site is expected to be incompatible with any engineered FRT variant that contains a mutation in the spacer region. The recombination event catalyzed by the FLP recombinase results in genetic modification, the nature of which is influenced by the relative orientation (direct or inverted), location and composition of the two target sites. The minimal 34bp FRT site can be used for inversion and excision reactions, while site-specific integration benefits from or requires the full 48bp FRT sequence, which contains an additional isolated base pair and a third direct repeat of the 13bp recombinase binding site (PMID:3879971).