RI-1 sensitizes cells to DNA damage by directly and specifically disrupting HsRAD51 and inhibiting the ability of RAD51 to form filaments on ssDNA. In addition, RI-1 alone generates single-agent toxicity in all three cancer cell lines (HeLa, MCF-7 and U2OS) with LD50 values in the 20–40 μM range. RI-1 decreases the rejoining of γ-H2AX foci in G2 phase cells and results in a higher level of unrepaired DSBs 6 hours after irradiation.