The purpose of the study is to evaluate whether an investigational drug is safe and effective for the treatment of the symptoms of Fragile X Syndrome. Eligible participants will receive up to 3 months of the investigational drug. During the study, they will be monitored for side effects and will meet with the doctor every 3 weeks. Between office visits, caregivers, parents, or guardians will also receive phone calls from the study doctor. A small payment will be offered to offset travel expenses. The investigational drug and study-related evaluations are provided at no cost. This study will last approximately 4 ½ months. For more information, please contact ChildResearch@umassmed.edu or call 508-856-5896.