RAD51 gene of S. cerevisiae is involved in DNA repair of double-strand breaks, mitotic and meiotic recombination. Rad51 protein is structurally similar to bacterial RecA proteins, which play crucial roles in recombination and DNA repair. Like RecA, Rad51 protein forms nucleofilament with a right handed helical structure, in which the DNA is extended 1.5 times than B-form DNA. Structural homologs have been cloned from various organisms including human and mouse. These results suggest that Rad51 is an essential recombination protein conserved from yeasts to human. Different from RecA, Rad51 protein requires at least an another protein, Rad52, to carry out recombination through the interaction between them.