Proteasome activity upon GSI-1 treatment The enzymatic activity of the 20S proteasome was measured after adding Bortezomib (20 nM), MG-132 (0.5 μM), GSI-1 (5 μM), DAPT (5 μM), Roscovitine (25 μM) and Fludarabine (100 μM) to cytoplasmic extracts from freshly isolated B-CLL patients. After 15 minutes incubation, the fluorogenic proteasome substrate LVVY-AMC was added. Monitoring the increase in fluorescence over time allowed quantification of the enzymatic activity.