Fragile X syndrome (FXS) is an inherited form of mental retardation that results from the loss of function of the fragile X mental retardation protein (FMRP). A recent report demonstrated alterations in the structure and plasticity of synapses on cerebellar Purkinje cells in Fmr1 knockout mice, which are a model of FXS. These synaptic alterations are associated with deficits in the cerebellar learning both in the mice and humans with FXS. This work forges an important link between the FMR1 gene, altered synaptic plasticity in the cerebellum and mental retardation.