Differential susceptibility of H-FT–transfected MEL clones to blockers of the PgP (MDR1) transporter. The various MEL clones were exposed to 20 μmol/L of verapamil or 5 μmol/L cyclosporin and assayed for metabolic activity 24 hours later by the Alamar Blue method, as described in Fig 3. The fluorescence intensity relative to the untreated control of each clone is depicted as a function of the treatment (± SE, n = 3), with * denoting statistically significant differences from control.