Biomarker Testing and DNA Collection in Subjects Participating in a Double-Blind, Placebo-Controlled, Crossover, Flexible-Dose Evaluation of the Efficacy, Safety, and Tolerability of STX209 in the Treatment of Irritability in Subjects With Fragile X Syndrome

The subjects that meet the inclusion and exclusion criteria and consent to participate in protocol 22001 will be offered participation in 22001A which will evaluate secreted protein before and after treatment with STX209 and placebo to determine if they correlate with effectiveness of treatment or susceptibility to treatment with STX209. These same subjects will also be asked to contribute a blood sample for DNA (deoxyribonucleic acid) collection. The investigators will study the DNA to determine if STX209 works better in people with specific gene variations, or to find new gene variations that predict how well STX209 works.