In Fragile X Syndrome, the silencing of the FMR1 gene leads to the reduced or absent production of FMRP, causing a disruption of synaptic structure and function: abnormally long, thin and immature dendritic spines have been observed in the brain cortex of Fragile X patients [38] and of Fmr1 gene knockout (Fmr1 KO) mice, a model of this disease [39]. Here, FMR1 is linked to fragile X syndrome.