Epileptic encephalopathies (EEs) are a devastating group of severe childhood onset epilepsies with medication resistant seizures and poor developmental outcomes. Many EEs have a genetic etiology and are often associated with de novo mutations in genes coding for proteins involved in synaptic transmission, including GABAA receptor subunit genes. A better understanding of GABAA receptor subunit mutations associated with EEs in vitro and in vivo will facilitate epilepsy diagnosis as well as treatments in the future. Here we employed a combination of next generation sequencing and in vitro functional assays and established for the first time that missense GABRG2 mutations are genetic risk factors for EEs. In addition, we focused on three nonsense GABRG2 mutations associated with epilepsies of different severities and demonstrated that they resulted in different structural disturbance and different suppression of wild-type partnering subunits. Finally we investigated the performance of heterozygous knock-in (KI) mice which bear the GABRB3(N110D) mutation associated with infantile spasms (Gabrb3+/N110D KI mice) in a battery of behavioral tasks, showing that they had significantly abnormal neurobehavioral profiles persisting into adulthood. To conclude, we have shown meaningful functional and structural changes for EE-associated GABRG2 mutations in vitro, and have determined the behavioral comorbidities of KI mice harboring a human infantile spasms GABRB3 mutation in vivo. Advisors/Committee Members: Kevin C. Ess (committee member), Robert L. Macdonald (committee member), Martin J. Gallagher (committee member), Douglas G. McMahon (Committee Chair).