There will be four study periods: Screening (up to 14 days in length), the Treatment Period (8 weeks), the Withdrawal Period (22 days), and Follow-up Period (up to 31 days). Subjects will be randomized to receive either STX209 (5 mg twice daily [BID], 10 mg BID or 10 mg three times daily [TID]) or placebo. Efficacy,safety and tolerability assessments will be performed periodically

Inclusion Criteria:

• Molecular documentation of the full FMR1 mutation
• Current pharmacological treatment regimen has been stable for at least 4 weeks prior to Screening.
• Subjects with a history of seizure disorder must currently be receiving treatment with antiepileptics and must have been seizure free for 6 months, or must be seizure free for 3 years if not currently receiving antiepileptics.
• If the subject is already receiving stable non-pharmacologic educational, behavioral, and/or dietary interventions, participation in these programs must have been continuous during the 2 months prior to Screening

Exclusion Criteria:

• Subjects with any condition, including alcohol and drug abuse, which might interfere with the conduct of the study, confound interpretation of the study results, or endanger their own well-being.
• Subjects who plan to initiate or change pharmacologic or non-pharmacologic interventions during the course of the study.
• Subjects who have taken another investigational drug within the last 30 days.
• Subjects who are not able to take oral medications