Broad drug resistance of CLL cells upon CD40 stimulation is not prevented by ERK inhibition. CLL cells were cocultured with control 3T3 (control) or CD40L-expressing cells for 48 hours, in the presence of ERK inhibitor PD98059 as indicated. After detachment and washing, cells were incubated with the indicated drugs as described in detail in “Methods,” and analyzed for apoptosis by MitoTracker staining after 24 hours (roscovitine, bortezomib, and GSI-1) or 48 hours (fludarabine). Cells cultured on 3T3 cells (■) are sensitive to all drugs, but CD40 stimulation (□) confers broad drug resistance and this is maintained when ERK is inhibited (). The data shown for untreated samples (medium) were measured at 24 hours. Apoptosis levels of medium samples at 48 hours were comparable.