ABC transporter substrates that are inhibitors do not increase transduction in T cells, hepatocytes, or nonhematopoietic progenitor cells. (A) Primary CD4⁺ T lymphocytes suboptimally transduced at 10 TU/cell alone or in the presence of 50 μg/mL verapamil or 50 μM ritonavir. Error bars indicate SD. (B) Rhodamine efflux from the lymphocytes as shown in panel A. Shown are time zero and 90 minutes. (C) Percentage transduction in neurosphere colonies after transduction at a total of 40 TU/cell alone or in the presence of 50 μg/mL verapamil, 100 μM diltiazem, or 50 μM quinidine. (D) Percentage transduction in mesenchymal progenitors (MSCs) after transduction at a total of 20 TU/cell alone or in the presence of 25 or 50 μg/mL verapamil, 100 μM diltiazem, or 50 μM quinidine. (E) Transduction efficiency in primary hepatocyte culture after addition of 10 TU/cell alone or in the presence of 25 or 50 μg/mL verapamil, 100 μm diltiazem, or 30 μM ritonavir.