The fragile X mental retardation protein (FMRP), the functional absence of which causes fragile X syndrome, is an RNA-binding protein that has been implicated in the regulation of local protein synthesis at the synapse. The mechanism of FMRP's interaction with its target mRNAs, however, has remained controversial. In one model, it has been proposed that BC1 RNA, a small nonprotein-coding RNA that localizes to synaptodendritic domains, operates as a requisite adaptor by specifically binding to both FMRP and, via direct base-pairing, to FMRP target mRNAs. Other models posit that FMRP interacts with its target mRNAs directly, i.e., in a BC1-independent manner. Here five laboratories independently set out to test the BC1-FMRP model. We report that specific BC1-FMRP interactions could be documented neither in vitro nor in vivo. Interactions between BC1 RNA and FMRP target mRNAs were determined to be of a nonspecific nature. Significantly, the association of FMRP with bona fide target mRNAs was independent of the presence of BC1 RNA in vivo. The combined experimental evidence is discordant with a proposed scenario in which BC1 RNA acts as a bridge between FMRP and its target mRNAs and rather supports a model in which BC1 RNA and FMRP are translational repressors that operate independently.fragile X syndrome ͉ non-protein-coding RNAs ͉ translational control S mall non-protein-coding RNAs perform important functions in the regulation of eukaryotic gene expression (1). In the mammalian central nervous system, they have been implicated in promoting organism-environment interactions (2). Small untranslated BC1 RNA is a translational repressor that is thought to participate in the regulation of local protein synthesis at the synapse (2, 3). BC1 RNA represses translation by targeting assembly of 48S initiation complexes (4). Interacting with eukaryotic initiation factor 4A (eIF4A) and poly(A) binding protein (PABP) (4-6), BC1 RNA prevents recruitment of the 43S preinitiation complex to the mRNA. Targets of BC1-mediated repression are those mRNAs that depend on the eIF4 family of factors for efficient initiation (4).Fragile X syndrome is caused by the functional absence of fragile X mental retardation protein (FMRP) (7,8). Consensus has developed over recent years that FMRP is, like BC1 RNA, a translational repressor that is active in postsynaptic microdomains (7, 9, 10). However, in contrast to BC1 RNA, FMRP is associated with polysomes (11-15), indicating that BC1 RNA and FMRP operate at different levels in the translation pathway.In an alternative model, it has been proposed that BC1 RNA and FMRP interact directly with each other (16,17). In this scenario, BC1 RNA (i) physically binds to FMRP, (ii) directly interacts, by base-pairing of its 5Ј domain, with select mRNAs that are FMRP targets, and (iii) acts as a bridge between FMRP and such target mRNAs, thus serving as a requisite adaptor (16, 17).The above two models cannot be reconciled. We, five independent groups with a longstanding interest in the molecular biology of...