FMR1 and fragile X syndrome: Individuals carrying intragenic loss of function mutations in the FMR1 gene show symptoms very similar to those carrying repeat expansions in the FM range (De Boulle et al., 1993; Gu et al., 1994; Lugenbeel et al., 1995; Myrick et al., 2014) and the disruption of the FMR1 gene in mice leads to the recapitulation of some aspects of FXS pathology including the increased density of immature dendritic spines (Comery et al., 1997).