Genetic recombination and the repair of double strand DNA breaks in Saccharomyces cerevisiae require Rad51, a homologue of the Escherichia coli RecA protein. In vitro, Rad51 binds DNA to form an extended nucleoprotein filament and catalyzes the ATP-dependent exchange of DNA between molecules with homologous sequences. Vertebrate Rad51 is essential for cell proliferation. The core reactions of homologous pairing, strand-transfer, exchange and annealing require RAD51 and its paralogs. These paralogs show high degree of divergence from RAD51 as well as from each other although the individual paralogs are well conserved between human and chicken cells. It has been suggested that Rad51 and Rad52 act cooperatively. In chicken RAD52 (-/-) cells, intrachromosomal recombination as measured by immunoglobulin gene conversion, and radiation-induced Rad51 nuclear focus formation (which is a putative intermediate step during recombinational repair), are unaffected. However, RAD52 has a clear role in genetic recombination because the frequency of targeted integration is reduced in RAD52 (-/-) cells. Although RAD52 can partially substitute for XRCC3 (one of the RAD51 paralogs), the rad52 xrcc3 double-mutant cells are non viable and exhibit extensive chromosomal breaks, indicating an overlapping (but non-reciprocal) role for RAD52 and XRCC3 in repairing DNA double-strand breaks. RAD54 interacts with RAD51 and plays an important role in DNA strand opening activities that occurs during strand invasion. RAD18 is also involved in the actual homologous recombination reaction along with its role in post-replication repair.