The human Rad51 protein (HsRad51) catalyzes homologous pairing and strand exchange between single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) during recombinational repair of double-stranded DNA breaks. An HsRad51 mutation that results in the substitution of Gln for Arg150 (R150Q) was found in bilateral breast cancer patients; however, the consequences of this R150Q mutation have not been elucidated. To determine how this HsRad51(R150Q) mutation affects HsRad51 function, in the present study, we purified the HsRad51(R150Q) mutant. The purified HsRad51(R150Q) was completely proficient in the ATP-hydrolyzing activity. A gel filtration analysis revealed that HsRad51(R150Q) also retained the polymer formation ability. In contrast, the ssDNA- and dsDNA-binding abilities of HsRad51(R150Q) were clearly reduced, as compared to those of HsRad51. These differences in the DNA-binding properties between HsRad51(R150Q) and HsRad51 may be important to account for the tumorigenesis in breast cancer patients with the HsRad51(R150Q) mutation.