Follow-up studies demonstrated that while a second calcineurin inhibitor did not have the same potentiating effect on FK866 in the cells, multiple other P-GP/ABCB1 inhibitors including verapamil and PGP-4008 recapitulated the effect of cyclosporin A on the potency of FK866 in both leukemia cells lines and multiple primary leukemia cells. This evidence concerns the gene ABCB1 and leukemia.