Functional expression of Pgp in K562 cells. (A) Expression of cell surface Pgp by HEL erythroleukemia cells, parental K562 cells, and sorted K562 infectants was measured by indirect immunofluorescence and flow cytometry using Mab U1C2. Neg refers to a control where an irrelevant isotype-matched Mab was used in place of U1C2. (B) Daunorubicin retention in the presence or absence of 3 μM verapamil was assayed by flow cytometry as described in “Materials and methods.” The rate of drug efflux is calculated as the slope of the line derived from the retention of daunorubicin over the assay period expressed as a percentage of that of inhibitor-treated cells for each time point (as described previously²⁴ ). The relative rate of efflux was calculated to be 0.152 for K562 (none), 0.4222 for K562 (Pgp⁺), and 0.117 for K562 (Bcl-x_L).