Effect of proteasome inhibitors on Mcl-1 degradation. (A) DLD1 cells were treated with DMSO, 1 μM bortezomib, 5 μM MG132, or 20 μM ALLN for 6 h, and then with 25 μg/ml of cycloheximide to block protein synthesis. Western blot analysis was performed. Data represent one of three independent experiments with similar results. (B) Quantitative analysis of the Western blot results shown in (A) using Optimas software. Data represent means ± SD of three assays.