P1cre::EBD encodes a ligand-inducible recombinase that is composed of the recombinase encoded by the cre locus of the bacteriophage P1 genome fused, often via a spacer sequence, to the estrogen-binding domain (EBD) from the Homo sapiens estrogen receptor gene. The presence of the estrogen-binding domain results in recombinase activity being dependent on the presence of estrogen, allowing recombinase activity levels to be fine-tuned to minimize toxicity that can be caused be chronic expression of the cre recombinase in transgenic flies (FBrf0139625). The length of any spacer sequence present may differ depending on the particular transgene or modified endogenous locus being used. Upon activation, P1cre::EBD mediates recombination between a pair of specific target sites, with compatible target sites corresponding to the wild-type loxP site and some of its derivatives. Recombination occurs between a pair of target sites oriented in the same direction; the 13bp repeats each act as binding sites for the recombinase, while the asymmetric 8bp spacer is the site of DNA strand exchange and determines the orientation of the target site (PMID:3856690). The recombination event catalyzed by P1cre::EBD 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 types of possible modification include deletion of DNA and generation of chromosomal rearrangements (reviewed in FBrf0231034).