Aim: HMGB 1 (high-mobility group box-1) is a nuclear protein containing a con- sensus RB (retinoblastoma)-binding LXCXE motif. In this study, we studied the potential association of HMGB 1 and RB and the in vitro and in vivo activities of HMGB 1 in human breast cancer cells. Methods: The protein-protein interaction was determined by immunoprecipitation-Weste blotting and glutathione-S-trans- ferase capture assays; cell growth and radiosensitivity were examined by cell counts, MTT assay, and clonogenic assay; cell cycle progression and apoptosis were evaluated using flow cytometry; and the antitumor activity of HMGB 1 was examined with tumor xenografts in nude mice. Results: HMGB 1 was associated with RB via a LXCXE motif-dependent mechanism. HMGB 1 enhanced the ability of RB for E2F and cyclin A transcription repression. The increased expression of HMGB 1 conferred an altered phenotypes characterized by the suppression of cell growth; G12 arrest and apoptosis was induced in MCF-7 cells containing the wild- type retinoblastoma (Rb) gene, but showed no activities in BT-549 cells contain- ing the Rb gene deletion. The HMGB 1-induced apoptosis accompanied by caspase 3 activation and PARP (poly(ADP-ribose)polymerase) cleavage. HMGB 1 elevated the radiosensitivity of breast cancer cells in both the MCF-7 and BT-549 cell lines. The enhanced expression of HMGB 1 caused a suppression of growth of MCF-7 tumor xenografts in nude mice, while LXCXE-defective HMGB 1 completely lost antitumor growth activity. Conclusion: HMGB 1 functions as a tumor suppressor and radiosensitizer in breast cancer. A HMGB 1-RB interaction is critical for the HMGB1-mediated transcriptional repression, cell growth inhibition, G12 cell cycle arrest, apoptosis induction, and tumor growth suppression, but is not required for radiosensitization. Therefore, it may be possible to design new therapies for the treatment of breast cancer that exert their effects by modulating the HMGB 1 and RB regulatory pathway and HMGB 1-related gene therapy.