Efficacy and Safety Study of STX209 (Arbaclofen) for Social Withdrawal in Adolescents and Adults With Fragile X Syndrome

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.