Multiple myeloma is an incurable cancer of plasma cells in the bone marrow. It has been found that arsenic trioxide (ATO) can kill multiple myeloma cells. The addition of ascorbic acid (vitamin C) can enhance ATO activity. Based on these findings, a clinical trial to test the safety and effectiveness of ATO and ascorbic acid for multiple myeloma was opened. The goals of this proposal are to identify correlates of response and resistance. These studies should provide insight into the factors that are important in determining clinical outcome, and will reveal targets for future therapies designed to cure multiple myeloma.