 This Phase 2 proof-of-concept study is a double-blind, randomized, placebo-controlled, crossover study to investigate ganaxolone treatment in children with fragile x syndrome FXS. Up to 60 subjects ages 6-17 yrs will be randomized to receive either ganaxolone or placebo for 6 weeks and then cross over to the opposite treatment for another 6 weeks. The aim of the study is assess the safety, tolerability and efficacy of ganaxolone for treatment of anxiety and attention in subjects with FXS. The hypothesis is that ganaxolone treatment compared to placebo will improve anxiety and attention as measured by the several neuropsychological and psychometric tests. 