 This is a single center study at UC Davis MIND Institute. Children with fragile x syndrome between the ages of 6-17yrs, inclusive will be randomized at a 1:1 ratio to receive ganaxolone or placebo treatment for 6 weeks, discontinue treatment and washout for 2 weeks, and then cross over to the opposite treatment for another 6 weeks. The primary aim of the study is to assess efficacy of ganaxolone treatment compared to placebo on clinical behaviors such as anxiety and attention as measured by Clinicians Global Impression-Improvement CGI-I. The key secondary efficacy measure is the Pediatric Anxiety Scale PARS. Other secondary efficacy measures include the visual analog scale VAS, Anxiety, Depression, Attention, and Mood Scale ADAMS, Swanson, Nolan, and Pelham-IV Questionnaire SNAP-IV, and Aberrant Behavior Checklist- Community Edition ABC-C. Tolerability and safety will be monitored by routine vital signs, physicalneurological exams, ECGs, clinical laboratory and adverse event assessments. 