Many evidences indicate that mice lacking the homeobox transcription factor engrailed-2 (En2(-/-) mice) represent a reliable model to investigate neurodevelopmental basis and gene expression changes relevant to autism spectrum disorders. Dysfunctions in fragile X mental retardation protein (FMRP), metabotropic glutamate receptor 5 (mGluR5), and GABAergic signaling pathways have been proposed as a possible pathogenic mechanism of autism spectrum disorders. Here, we exploited En2(-/-) mice to investigate hippocampal expression of FMRP, mGluR5, and GABA(A) receptor β3 subunit (GABRB3). Quantitative reverse-transcription PCR showed that all these mRNAs were significantly downregulated in En2(-/-) mice compared with wild-type littermates. Western blot and immunohistochemistry confirmed the downregulation of FMRP and GABRB3 proteins, while showing a significant increase of mGluR5 protein in the En2(-/-) hippocampus. Our results suggest that the dysregulation of FMRP-mGluR5 signaling pathway, accompanied with a downregulation of GABRB3 expression, may contribute to the 'autistic-like' features observed in En2 mice, providing possible molecular targets for future pharmacological studies.