mRNA localization and regulated translation play central roles in neurite outgrowth and synaptic plasticity. A key molecule in these processes is the Fragile X mental retardation protein, FMRP, which is involved in the metabolism of neuronal mRNAs. Absence or mutation of FMRP leads to spine dysmorphogenesis and impairs synaptic plasticity. Studies that have mainly been performed on the mouse and Drosophila models for Fragile X Syndrome showed that FMRP is involved in translational regulation at synapses, but even 15 years after discovery of the FMR1 gene, the precise working mechanisms remain elusive.