Fragile X-syndrome is caused by functional inactivation ofthe Fmri gene, and represents the most common genetic form of intellectual disability. However, the mechanisms of fragile X-syndrome pathogenesis are incompletely understood. As a result, few potential therapeutic avenues to treat the disorder are available. Based on observations that different forms of synaptic plasticity, most prominently mGluR5-dependent LTD and retinoic acid-dependent homeostatic plasticity, are blocked in Fmri knockout mice, the present project is led by the overall hypothesis that fragile X-syndrome involves an impairment of experience-driven synaptic excitation/inhibition (E/l) adjustments. Guided by this hypothesis, we propose four specific aims that explore the nature and developmental dynamics of FXS pathogenesis in mouse models using conditional and constitutive Fmri gene inactivation and a combination of biochemical, physiological, and behavioral assays, with a focus on activity- and experience-induced changes in the synaptic excitatory/inhibitory (E/l) state. Additionally, we propose to investigate whether activating oxytocin signaling can restore aspects ofthe altered E/l state in the hippocampal circuitry of FXS mice. With these experiments, we aim to establish in a mouse model how synaptic dysfunction, especially that related to synaptic E/l imbalance, is linked to the behavioral defects in FXS, and to obtain a comprehensive understanding ofthe development of FXS-related pathology. These studies will lead to a better and more comprehensive understanding of fragile X-syndrome and define disease mechanisms that could lead to the identification of potential therapeutic targets.