BAL101553 has demonstrated broad anti-tumour activity across a panel of cell lines and xenograft models.11–20 Importantly, BAL101553 is active in human cancer models that are refractory to standard of care therapeutics, including clinically relevant MTAs, due to P-glycoprotein overexpression and non-P-glycoprotein-related mechanisms.14,15,20 Hence, the activity of BAL101553 does not appear to be affected by factors that confer resistance to conventional MTAs. This evidence concerns the gene ABCB1 and cancer.