RSR corresponds to the recombinase encoded by the pSR1 plasmid of Zygosaccharomyces rouxii. It recognizes the RSRT target site, with recombination occurring between a pair of these sites. The 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. The types of possible modification include deletion of DNA, generation of chromosomal rearrangements and integration of DNA into the genome. RSR also shows some cross-reactivity with the B3RT target site, showing approximately one-third of the level of recombinase activity towards this site compared to RSRT in an in vivo excision assay in transgenic Drosophila. RSR shows low toxicity in transgenic Drosophila (FBrf0214794).