Safety, Tolerability and Efficacy Study of STX209 in Subjects With Fragile X Syndrome

Inclusion Criteria: * Male or female subjects 12 to 40 years of age eventually expanding to 6 years of age * Molecular documentation of the fragile X mutation. * Clinical Global Impression - Severity (CGI-S) rating for problem behavior of moderate or higher at screening and at Visit 1 * An Aberrant Behavior Checklist (ABC-C) Irritability Subscale score \>12 and at least 3 items on the Irritability Subscale rated at least moderate or above. * Current treatment with no more than three psychoactive medications, including anti-epileptics. * Current pharmacological treatment regimen has been stable for at least 4 weeks. Exclusion Criteria: * Subjects with a history of seizure disorder who are not currently receiving treatment with antiepileptics. * 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. This includes, but is not limited to impairment of renal function, evidence or history of malignancy or any significant hematological, endocrine, cardiovascular, respiratory, hepatic, or gastrointestinal disease. * Subjects who plan to initiate or change pharmacologic or non-pharmacologic interventions during the course of the study. * Subjects who are currently receiving treatment with racemic baclofen. * Subjects currently treated with vigabatrin or tiagabine. * Subjects taking another investigational drug currently or within the last 30 days.