A de novo mutation in the gene, which encodes the Kv4.2 K channel, was identified in twin boys with intractable, infant-onset epilepsy and autism. Kv4.2 channels undergo closed-state inactivation (CSI), a mechanism by which channels inactivate without opening during subthreshold depolarizations. CSI dynamically modulates neuronal excitability and action potential back propagation in response to excitatory synaptic input, controlling Ca influx into dendrites and regulating spike timing-dependent plasticity. Here, we show that the V404M mutation specifically affects the mechanism of CSI, enhancing the inactivation of channels that have not opened while dramatically impairing the inactivation of channels that have opened. The mutation gives rise to these opposing effects by increasing the stability of the inactivated state and in parallel, profoundly slowing the closure of open channels, which according to our data, is required for CSI. The larger volume of methionine compared with valine is a major factor underlying altered inactivation gating. Our results suggest that V404M increases the strength of the physical interaction between the pore gate and the voltage sensor regardless of whether the gate is open or closed. Furthermore, in contrast to previous proposals, our data strongly suggest that physical coupling between the voltage sensor and the pore gate is maintained in the inactivated state. The state-dependent effects of V404M on CSI are expected to disturb the regulation of neuronal excitability and the induction of spike timing-dependent plasticity. Our results strongly support a role for altered CSI gating in the etiology of epilepsy and autism in the affected twins.