To explore the efficacy, safety and tolerability of STX209 (arbaclofen) administered for the treatment of social withdrawal in adolescents and adults with fragile X syndrome (FXS)

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.