In our experiments, we used the following three variants of murine leukemia cells: (i) the original ABCB1-negative L1210 cells (S), (ii) ABCB1-positive cells (R) in which ABCB1 expression was induced by vincristine [24] and (iii) ABCB1-positive (T) cells in which ABCB1 expression was induced by transfection with the human gene encoding ABCB1 [25]. Here, ABCB1 is linked to leukemia.