SCN1A and epilepsy: Mutations or deletion of Nav1.1 lead to the loss of sustained high-frequency firing of action potential and excitability in the hippocampal and cortical inhibitory interneurons and Purkinje neurons, which allows hyperexcitability of principal neurons (e.g., dentate granule cells and pyramidal neurons) in the neuronal networks, thus leading to brain hyperexcitability and subsequently epilepsy.