Basal levels of FosB/ΔFosB immunoreactivity in the medial geniculate body and primary acoustic area were similar in both genotypes, but a different scenario emerged in the hippocampal formation, in which FosB/ΔFosB immunoreactivity was unexpectedly higher in unstimulated P45 Fmr1 KO mice compared to age-matched WTs, suggesting that regions notoriously involved in cognitive and emotional physiological functions (Koe et al., 2009; Herry et al., 2010) were hyperactive in our animal model of FXS. This evidence concerns the gene FMR1 and fragile X syndrome.