The purpose of this study is to see if the investigational drug, selinexor, has any effects in eligible participants with advanced unresectable dedifferentiated liposarcoma (DDLS). Selinexor will be given at a fixed oral dose of 60mg or placebo twice weekly on Days 1 and 3 during Weeks 1-6 of each 42 day cycle until disease progression, intolerability, consent withdrawal or unblinding in Phase 2, 3, and open-label portion of the study.

You may be eligible for this study if you meet the following criteria:

• Conditions: Liposarcoma
• Age: 18 years - 99 years
• Gender: All