Double-strand breaks (DSBs) and single-strand gaps in damaged DNA are efficiently repaired by mechanisms associated with recombination. Recombination is a series of complex biochemical reactions, requiring at least 20 gene products, even in Escherichia coli. Genes homologous to bacterial and yeast recombination genes have been cloned in higher eukaryotes, suggesting there might be a common fundamental mechanism of recombination among a wide variety of species. In eukaryotes, protein-protein interactions play important roles in recombination: by interacting with a specific protein(s), the complex involved in repair of DSBs is modified to carry out specialized cellular functions, such as meiotic recombination and switching of mating types in yeast.