Flow cytometric analysis of MDR1 Pgp activity in B lymphoblasts. B lymphoblasts from a patient with Scott syndrome and normal controls were loaded with Rh123 (10 minutes, 37°C), washed and reincubated in Rh123-free medium in the presence (3 hours + Ver) or absence (3 hours) of 50 μmol/L verapamil. Aliquots were removed at t = 0 hour and 3 hours, and analyzed by flow cytometry. Dot plots are shown for one normal control (C2; lower panels) and one Scott lymphoblast cell line (S2; upper panels). An arbitrary gate is drawn to facilitate comparison between samples. All three Scott and three normal control cell lines displayed MDR1 activity in the absence of verapamil, although differences in activity between cell lines were observed (not shown).