GRM5 and fragile X syndrome: Other groups have reported that a single dose treatment of gaboxadol significantly reversed several phenotypes in preclinical FXS mouse models, including auditory startle response, hyperactivity, stereotypy and aggression.23,32-34 This drug class has however previously demonstrated pharmacoresistance and loss of efficacy following chronic dosing in other disorders.35,36 In addition to this, several mGluR5-negative allosteric modulators have also shown treatment resistance following chronic dosing in FXS mice.