FMRP, lack of which causes Fragile X syndrome, is an RNA-binding protein which can regulate the translation of a subset of mRNAs (Miyashiro et al., Neuron, 2003; Weiler et al., PNAS, 2004). While expression of individual mRNAs may be either higher or lower in FMRP-KO mice, the mechanism of translational regulation by FMRP is still unknown. In response to various sorts of stress (heat, chemical or injury stress) and synaptic stimulation, FMRP-containing “stress granules” form and FMRP shuttles between stress granules and polyribosomes (Mazroui et al., Hum Mol Genet, 2002, Weiler et al., SfN 2003, Kim et al., SfN 2004). We now report that stress granules contain at least 3 mRNAs which are known to be bound by FMRP: fmr1, adenylate cyclase 3, and glucocorticoid receptor. We also found that stress granules include such ribosomal proteins as S6, L4, L7a, but not L3, L28, which suggests that they do not harbor complete ribosomes like RNA granules previously described by Aschrafi et al., PNAS, 2005. These observa...