PGP and leukemia: The following variants of mice leukemia L1210 cells were used as cell models: (i) P-gp-negative parental cells (S); (ii) P-gp-positive-R cells obtained by the selection of S cells in medium containing vincristine (VCR) in stepwise increasing concentrations [15]; and (iii) P-gp-positive T cells obtained by the transfection of S cells with the gene encoding full length P-gp [16].