Combination of histone deacetylase inhibitors (HDACIs) and TRAIL. HDACIs, such as sodium butyrate, trichostatin A (TSA), and suberoylanilide hydroxamic acid (SAHA) up-regulate DR5 expression through transcription, resulting in an increased level of the DR5 protein on the cell membrane and, consequently, an increased chance that TRAIL binds to DR5. The combination of HDACI and TRAIL can synergistically induce apoptosis in malignant tumor cells