Ring finger protein 8 (RNF8)， an FHA/RING domain containing E3 ubiquitin ligase， is critical in supporting genome integrity by facilitating the assembly of multiple DNA repair proteins at DNA lesions following DNA damage. In the present study， a search for novel binding partners of RNF8 was performed using a yeast two?hybrid screening assay， and small ubiquitin?like modifier (SUMO)2/3 was identified as one of the major RNF8?binding candidates. GST pull?down and immunoprecipitation assays revealed that RNF8 bound directly and noncovalently to SUMO2/3， but not to SUMO1， and that the FHA domain of RNF8 was required for the binding to SUMO2/3. Furthermore， RNF8 co?localized with SUMO2/3 at sites of DNA lesions in response to ionizing radiation， as revealed by immunofluorescence assay. Survival assay indicated that the depletion of RNF8 and SUMO2/3 resulted in decreased cellular resistance to genotoxic stress. These data suggested that the binding of RNF8 to SUMO2/3 promoted the response to DNA damage.