APP and fragile X syndrome: Consistent with these findings, Aβ levels are elevated in the brain of Fmr1KO mice, and several FXS phenotypes including mGluR-LTD can be rescued by genetically reducing APP and Aβ levels (Fmr1KO/AppHET mice; Westmark and Malter, 2007; Westmark et al., 2011b).