PGP and lymphoid leukemia: As experimental models, parental leukemia cell lines L1210 (murine lymphocytic leukemia cells), along with MOLM-13 and SKM-1 (both human acute myeloid leukemia cells), with limitingly low P-gp expression, and their P-gp positive variants, R (L1210), SKM-1/vcr, and MOLM-13/vcr, obtained by selection/adaptation via passaging in media with gradually increasing concentrations of vincristine [19], were used.