GABBR1 and fragile X syndrome: However, contrary to what was predicted by a decade of studies in FXS animal models, administration of mavoglurant, an mGluR5 antagonist68, or arbaclofen, a GABA B receptor agonist69,70, to adolescents and adults with FXS showed no significant improvement in behavioral traits in a randomized, double-blind, placebo-controlled phase 2 trial.