The GABA(A) receptor gamma2 subunit mutation, Q351X, associated with generalized epilepsy with febrile seizures plus (GEFS+), created a loss of function with homozygous expression. However, heterozygous gamma2(+/-) gene deletion mice are seizure free, suggesting that the loss of one GABRG2 allele alone in heterozygous patients may not be sufficient to produce epilepsy. Here we show that the mutant gamma2 subunit was immature and retained in the endoplasmic reticulum (ER). With heterozygous coexpression of gamma2S/gamma2S(Q351X) subunits and alpha1 and beta2 subunits, the trafficking deficient mutant gamma2 subunit reduced trafficking of wild-type partnering subunits, which was not seen in the hemizygous gene deletion control. Consequently, the function of the heterozygous receptor channel was reduced to less than the hemizygous control and to less than half of the wild-type receptors with a full gene dose. Pulse-chase experiments demonstrated that in the presence of the mutant gamma2S(Q351X) subunit, wild-type alpha1 subunits degraded more substantially within 1 h of translation. We showed that the basis for this dominant-negative effect on wild-type receptors was due to an interaction between mutant and wild-type subunits. The mutant subunit oligomerized with wild-type subunits and trapped them in the ER, subjecting them to glycosylation arrest and ER-associated degradation (ERAD) through the ubiquitin proteosome system. Thus, we hypothesize that a likely explanation for the GEFS+ phenotype is a dominant-negative suppression of wild-type receptors by the mutant gamma2S subunit in combination with loss of mutant gamma2S subunit protein function.