The voltage-gated Na ؉ (Nav) channel provides the basis for electrical excitability in the brain. This channel is regulated by a number of accessory proteins including fibroblast growth factor 14 (FGF14), a member of the intracellular FGF family. In addition to forming homodimers, FGF14 binds directly to the Nav1.6 channel C-tail, regulating channel gating and expression, properties that are required for intrinsic excitability in neurons. Seeking amino acid residues with unique roles at the protein-protein interaction interface (PPI) of FGF14⅐Nav1.6, we engineered model-guided mutations of FGF14 and validated their impact on the FGF14⅐Nav1.6 complex and the FGF14: FGF14 dimer formation using a luciferase assay. Divergence was found in the -9 sheet of FGF14 where an alanine (Ala) mutation of Val-160 impaired binding to Nav1.6 but had no effect on FGF14:FGF14 dimer formation. . Altogether these studies indicate that the -9 sheet and the N terminus of FGF14 are well positioned targets for drug development of PPI-based allosteric modulators of Nav channels.Voltage-gated sodium (Nav) 2 channels are responsible for the initiation and propagation of the action potential in excitable cells. Nine isoforms of Nav channels (Nav1.1-Nav1.9) have been characterized functionally, and evidence for a tenth one (Na x ) has been provided (1-12). Nav channels are differentially expressed in organs, with Nav1.1, -1.2, -1.3, and -1.6 found primarily in the central and peripheral nervous systems, Nav1.4 in the adult skeletal muscle, Nav1.5 in cardiac muscle, and Nav1.7, -1.8, and -1.9 primarily in the peripheral nervous system (3,4,7,12,13). With such widespread expression, it is not surprising that numerous diseases have been ascribed to mutations of specific Nav channel isoforms (4, 14). These include the Dravet syndrome and other types of epilepsy (15-17); pain-related syndromes, such as congenital insensitivity to pain (18, 19), primary erythromelalgia (20), and paroxysmal extreme pain disorder (21, 22); and cardiac arrhythmias with congenital long QT syndrome (LQTS) type 3 (23, 24); and Brugada syndrome (25). Furthermore, SNPs and/or copy variants within Nav channel genes have been associated recently with autism (Nav1.2) (26). Nav channels blockers are currently used in combined therapy for bipolar disorder (27, 28), depression (29, 30), and schizophrenia (31), extending the role of Nav channels to virtually all brain disorders both neurological and psychiatric (14,26,32). Their centrality in the pathophysiology of so many disruptive diseases has made Nav channels key pharmacological target sites for antiepileptic, analgesic, antiarrhythmic, and psychiatric drugs (11,14,33,34). Unfortunately, current Nav channel blockers lack specificity, as they are directed against molecular domains conserved across all Nav isoforms. As such, therapies based on these medications can result in severe side effects, such as Stevens-Johnson syndrome, blood dyscrasias, and ataxia (35). Although some success has been achieved in developing more ta...