RAD51 and cancer: • Inhibits HR DNA repair and RAD51 foci formation in cancer cells induced by DNA damage.• Single agent activity in TNBC cells and enhances the sensitivity of BRCA-proficient TNBC cells to the PARPi, olaparib through synthetic lethality.• Enhances radiosensitivity in combination with olaparib in different cancer cells by inducing synthetic lethality. IC50 = 2.6-11.9 μM