(A) Correlation between Pgp upregulation and increase in Pgp function in Pgp-negative blasts obtained from 10 patients. The data points represent 32 independent drug treatments for which Pgp expression and function measuments were performed as described in Table1. Pgp expression or function was expressed as the ratio of the arithmetic mean of fluorescence (MCF) of MRK 16 versus the IgG_2a control or Rh123 fluorescence in the presence or absence of 2 μmol/L PSC 833, respectively, as described in Materials and Methods. (B) Correlation between Pgp upregulation and increase in Pgp function in Pgp-positive blasts obtained from 9 patients. The data points represent 22 independent drug treatments for which Pgp expression and function measuments were performed as described in Table2. Pgp expression or function was expressed as the ratio of the MCF of MRK 16 versus the IgG_2a control or Rh123 fluorescence in the presence or absence of 2 μmol/L PSC 833 respectively as described in Materials and Methods. (□) Pgp ratio/Rh 123 ratio.