Generalized epilepsy with febrile seizures plus type 1 is an inherited human epileptic syndrome, associated with a cysteine-to-tryptophan (C121W) mutation in the extracellular immunoglobin domain of the auxiliary β1 subunit of the voltage-gated sodium channel. The mutation disrupts β1 function, but how this leads to epilepsy is not understood. In this study, we make several observations that may be relevant for understanding why this β1 mutation results in seizures. First, using electrophysiological recordings from mammalian cell lines, coexpressing sodium channel α subunits and either wild-type β1 or C121Wβ1, we show that loss of β1 functional modulation, caused by the C121W mutation, leads to increased sodium channel availability at hyperpolarized membrane potentials and reduced sodium channel rundown during high-frequency channel activity, compared with channels coexpressed with wild-type β1. In contrast, neither wild-type β1 nor C121Wβ1 significantly affected sodium current time course or the voltage dependence of channel activation. We also show, using a Drosophila S2 cell adhesion assay, that the C121W mutation disrupts β1–β1 homophilic cell adhesion, suggesting that the mutation may alter the ability of β1 to mediate protein–protein interactions critical for sodium channel localization. Finally, we demonstrate that neither functional modulation nor cell adhesion mediated by wild-type β1 is occluded by coexpression of C121Wβ1, arguing against the idea that the mutant β1 acts as a dominant-negative subunit. Together, these data suggest that C121Wβ1 causes subtle effects on channel function and subcellular distribution that bias neurons toward hyperexcitabity and epileptogenesis.