Recombinases are genetic recombination enzymes. DNA recombinases are widely used in multicellular organisms to manipulate the structure of genomes, and to control gene expression. These enzymes, derived from bacteria and fungi, catalyze directionally sensitive DNA exchange reactions between short (30–40 nucleotides) target site sequences that are specific to each recombinase. These reactions enable four basic functional modules, excision/insertion, inversion, translocation and cassette exchange, which have been used individually or combined in a wide range of configurations to control gene expression.