Tumor cells remain viable after a combination treatment of proteasome inhibitor and sub-lethal irradiation. Tumor cells were mock-irradiated (0 Gy) or irradiated with 5 Gy and cultured for 24 h. Following incubation, mock-irradiated or irradiated cells were treated with 10 nM bortezomib and incubated for an additional 24 h. An Annexin V-PE Apoptosis Detection Kit I (BD PharMingen, San Diego, CA, USA) was used for staining; results were quantified by Flow cytometry analysis and were analyzed using FlowJo software (FlowJo LLC, Ashland, OR, USA). Experiment was repeated three times with similar results. The relative increase of dead tumor cells in (A) SW620 (B) and HTC116 colorectal cancer cells.