The DNA repair proteins Rad50, Rad51A-D, Rad52 and Rad54 A and B are key components of the double-strand break repair pathway (DSBR). The process of homologous recombination is a major DNA repair pathway that operates on DNA double-strand breaks to promote error-free repair. Human Rad51 protein assembles on a DNA molecule through cooperative binding and forms a long filament for homologous recombination. In addition to Rad51 protein, five paralogs have been identified in vertebrates, Rad51B, Rad51C, Rad51D, Xrcc2 and Xrcc3. Rad51B is a widely expressed protein that is involved in the repair of double-stranded DNA breaks that can arise during DNA replication. It can form a dimer with Rad51C and this complex shows single-stranded DNAdependent ATPase activity.