The fragile X syndrome results from amplification of a polymorphic CGG repeat in the 5' untranslated region of the FMR1 gene and is the most common form of inherited mental retardation. Although the lack of expression of FMR1 protein produces the fragile X syndrome, the pathological mechanism is not known. FMR1 is an RNA binding protein that performs a chaperone or transporter function. It interacts with FXR1 and FXR2. These proteins have 60% identity with FMR1 and have similar KH and RNA binding domains. All three proteins are cytosolic, associate with the 60S ribosomal subunit, and form homomers or heteromers with each other. During embryonic development, FMR1, FXR1, and FXR2 are expressed primarily in the nervous system. Small amounts of FMR1, FXR1, and FXR2 shuttle between the cytoplasm and nucleus. FMR1 is found in the nucleoplasm, while FXR2 is localized to the nucleolus. Thus, FMR1, FXR1, and FXR2, either individually or in tandem, function to transport RNAs throughout the cytoplasm and to specific sites in the nucleus. Alterations in the interactions between these proteins and ribosomal components may underlie the pathology that produces fragile X syndrome.