Additional Study Information: This is a Phase 2-3 study evaluating the effects of selinexor in patients with advanced unresectable dedifferentiated liposarcoma (DDLS). This study drug selinexor works by trapping tumor suppressing proteins within the nucleus of the cells and thus causing the cancer cells to die or stop growing. Selinexor has previously been tested in humans to define a safe dose to be administered. It is not known at this time if selinexor will treat your cancer. Selinexor is currently being tested in other clinical trials in patients with advanced cancers. This study will examine the effects of selinexor on your cancer and on your body including any side-effects that you may experience.