The Fragile X syndrome, which results from the absence of functional FMRP protein, is the most common heritable form of mental retardation. Here, we show that FMRP acts as a translational repressor of specific mRNAs at synapses. Interestingly, FMRP associates not only with these target mRNAs, but also with the dendritic, non-translatable RNA BC1. Blocking of BC1 inhibits the interaction of FMRP with its target mRNAs. Furthermore, BC1 binds directly to FMRP and can also associate, in the absence of any protein, with the mRNAs regulated by FMRP. This suggests a mechanism where BC1 could determine the specificity of FMRP function by linking the regulated mRNAs and FMRP. Thus, when FMRP is not present, loss of translational repression of specific mRNAs at synapses could result in synaptic dysfunction phenotype of Fragile X patients.