We hypothesized that, in addition to chemosensitizing leukemia cells overexpressing ABCG2 to ABCG2 substrate drugs, quizartinib, which is orally administered, would inhibit ABCG2 on intestinal mucosal cells, and thus increase absorption of co-administered oral ABCG2 substrate drugs, including those with the potential to contribute to QT prolongation. The gene discussed is ABCG2; the disease is leukemia.