During the last years, homologues of E coli RecA have been cloned in numerous species including man. These Rad51 proteins share sequence as well as functional homologies with the bacterial protein. Human Rad51 (HsRad51) is able to catalyze strand exchange in vitro between homologous DNAs, but with a lower efficiency compared to that of RecA. This suggests the requirement of additional factors. A very interesting feature of Rad51 is its essential role in mouse which could mean that it has gained an essential function in cell growth. The interaction of HsRad51 with several tumor suppressor genes namely p53, BRCA1 and BRCA2 implies possible role(s) of this protein in tumorigenesis. Thus, the continued study of Rad51 should bring important insights not only into homologous recombination mechanisms but also into cell proliferation regulation.