This study is testing if a drug called selinexor can help people with certain types of tumors that have come back or are hard to treat. A tumor is a group of cells that grow too fast and can form lumps or masses. Wilms tumor is a rare kidney cancer mostly found in children, while rhabdoid tumors and MPNST (malignant peripheral nerve sheath tumors) are other types of solid tumors. The study looks at tumors that make too much of a protein called XPO1 or have changes in their genes that make XPO1 more active.

• Participants must be between the ages of 1 and 6 years old and have specific types of tumors.
• The study will involve taking selinexor and undergoing tests to see how well it works.
• People with certain health conditions or who have used similar drugs before cannot join.