Fluorescence histograms for paired T-cell ALL specimens at diagnosis and relapse. Paired blast samples from a patient with T-ALL (specimens T13 and rT6 in Table 2 and 3, respectively; right panel) were assayed for DHFR and MTX transport with PT430 and flow cytometry as described in Materials and Methods. Fluorescence histograms are shown for blasts treated with PT430 (20 μmol/L) and suspended in drug-free medium (No Additions) or in the presence of Mtx (0.5 μmol/L) or trimetrexate (TMQ; 0.5 μmol/L). Data are plotted as computer-generated histograms after the analysis of 10,000 cells. The abscissa is a logarithmic scale of the relative fluorescence intensity in a given fluorescence intensity channel.