A Study of the Efficacy, Safety, and Tolerability of STX209 (Arbaclofen) for the Treatment of 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.