FMR1 and fragile X syndrome: Moreover, a mouse model of FXS which lacks FMRP (Fmr1 KO) exhibit higher basal levels of APP, and of the pathogenic product of APP cleavage, β-amyloid (Aβ), and FXS patients show abnormal Aβ levels in plasma and brain tissues (Westmark and Malter, 2007; Westmark et al., 2011).