The investigators therefore hypothesized that a selective XPO1 inhibitor would show efficacy with an acceptable therapeutic index in CLL and other diseases. Indeed, XPO1 inhibition in normal cells (ie, possessing an intact genome) leads to transient cell cycle arrest without cytotoxicity, followed by fast recovery after the drug is removed. To date, efforts to clinically pharmacologically inhibit XPO1 have been unsuccessful because of off-target effects. A selective XPO1 antagonist may allow targeting of the TSPs axes in tumor cells.