The coordinate decrease in (a) es-specific 5-(Sx8)-F fluorescence and (b) increase in viability of araC-treated cells, in the presence of graded concentrations of NBMPR. Es transporter sites were fractionally occupied by treatment of cells with graded concentrations of NBMPR, before staining with 5-(Sx8)-F and analysis by flow cytometry, as described in the legend to Fig 4. 5-(Sx8)-F concentrations, known to be sufficient to saturate es transporter sites, were 30 nmol/L (OCI/AML-2), 20 nmol/L (patient H.M.), 80 nmol/L (CEM), and 20 nmol/L (patient T.Z.). Es-specific staining with 5-(Sx8)-F is shown (open symbols), as is the viability of cells treated with NBMPR at the same, graded concentrations, measured with the MTT assay after a 96-hour exposure to araC (closed symbols). AraC concentrations were 8 nmol/L (OCI/AML-2 cells, [A]), 300 nmol/L (blasts from AML patient H.M., [B]), 1 nmol/L (CEM cells, [C]), and 100 nmol/L (blasts from ALL patient T.Z., [D]). Those concentrations reduced cell viability to 3.8%, 17%, 2.1%, and 50% of controls in the absence of NBMPR, respectively, in the four cell types. In each panel, flow cytometry data are means ± SD of duplicate determinations, and cytotoxicity values are means ± SEM of 6 to 12 replicates.