The fragile X mental retardation 1 protein (FMR1) is a cytoplasmic RNA binding protein abundantly expressed in the brain neurons. The absence of FMR1 protein leads to fragile X syndrome associated with the expansion of trinucleotide 5′-d(CGG)-3′ repeats within the FMR1 gene. Three RNA binding domains including 1 RGG box and 2 KH domains were found in FMR1. Besides the presence of RNA binding motifs, FMR1 was also found to carry nuclear localization and nuclear export signals and it is believed to shuttle in and out the nucleus. FMR1 associates with polyribosomes through RNA and it is suggested to act as both a negative regulator and an activator of mRNA translation. Furthermore, recent studies showed that FMR1 can associate with miRNA and the components of the miRNA pathway in vivo.