• Inhibits DSB-induced HR DNA repair and RAD51 foci formation induced by DNA damage.• Enhances sensitivity of cancer cells to IR, MMC, cisplatin, etoposide and topotecan.• Significantly increases sensitivity of doxorubicin in myeloma cells and MMS in combination with PARPi in MEF cells by triggering synthetic lethality. The gene discussed is RAD51; the disease is plasma cell myeloma.