<qa>  What is the duration of the study period?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 