Although fragile X syndrome is caused by the absence of fragile X gene expression, little is known about the pathogenic processes underlying the mental retardation. Recent findings that the fragile X protein, FMRP, contains RNA binding motifs and nuclear transport signals and associates with ribosomes suggest that FMRP may be involved in either mRNA processing, transport, or translation. To test the hypothesis that absence of FMRP may affect the processing of specific transcripts, we have used an RNA differential display assay (RDDA) to identify differentially expressed transcripts in lymphoblast lines derived from fragile X syndrome patients. A 0.9-kb cDNA fragment that showed reduced expression in a fragile X lymphoblast cell line was found to be identical to G3BP (Ras-GTPase-Activating protein SH3-domain-binding protein). Quantitative reverse transcriptase-polymerase chain reaction showed that the expressed levels of G3BP mRNA in fragile X lymphoblast cell lines were significantly less than controls. Our results indicate that G3BP mRNA may be regulated by FMRP and supports the hypothesis that FMRP may modulate the transcription of specific transcripts.