B3RT is the target site for the B3R recombinase. It is composed of two 14bp inverted repeats flanking a 5bp spacer sequence: 5'-GGTTGCTTAAGAATaagtaATTCTTAAGCAACC-3'. B3R mediates recombination between a pair of B3RT sites. This recombination event results in genetic modification, the nature of which is influenced by the relative orientation (direct or inverted), location and composition of the two recognition sites (FBrf0214794).