GRIA1 and fragile X syndrome: Furthermore, Fmr1−/− mice (the fragile X syndrome mouse model) show neuronal hyperexcitability, because loss-of-function dephosphorylation of NEDD4-2 impairs NEDD4-2-GluA1 interaction and GluA1 ubiquitination after chronic activity stimulation, but not under physiological condition24.