The present invention relates to crystalline forms of the compound represented by Structural Formula I, and compositions comprising crystalline forms of the compound represented by Structural Formula I described herein. The crystalline forms of the compound of Structural Formula I and compositions comprising the crystalline forms of the compound of Structural Formula I provided herein, in particular, single crystalline Form A, can be incorporated into pharmaceutical compositions, which can be used to treat various disorders associated with CRM1 activity, including cancer. Also described herein are methods for preparing the compound of Structural Formula I and its single crystalline forms.