Multifunctional polyribosome-associated RNA-binding protein that plays a central role in neuronal development and synaptic plasticity through the regulation of alternative mRNA splicing, mRNA stability, mRNA dendritic transport and postsynaptic local protein synthesis of a subset of mRNAs. Binds poly(G) and poly(U), and to a lower extent poly(A) and poly(C). Homodimer. Heterodimer. Expressed ubiquitously throughout the embryo. Shows tissue-specific expression in adults, being abundant in most neurons of the central nervous system (CNS) and in all spermatogenic cells of the testis. Belongs to the FMR1 family.