Microtubule-binding protein which directly binds tubulin and is involved in both polymerization and stabilization of microtubules (PubMed:22726441). Through its action on microtubules, may participate to the refinement of axons by negatively regulating axonal and leading processes branching (PubMed:22726441). Plays a crucial role in neuron polarization and migration in the cerebral cortex and the hippocampus (PubMed:22726441). Regulates voltage-gated sodium channels transport and function (PubMed:21817159). May also play a role in MAPK signaling (PubMed:11378392, PubMed:12244047). Required for the development of axonal initial segment-targeting inhibitory GABAergic synapses made by chandelier neurons (PubMed:30679375). Seems not to be involved in neuroblast polarization and migration but regulates axon branching. Interacts with SCN8A; regulates SCN8A activity (By similarity). Interacts with SCN1A; may regulate SCN1A activity (By similarity). Interacts with SCN5A; the interaction is direct and may regulate SNC5A density at membranes and function (PubMed:21817159). May also interact with SCN2A and SCN11A (By similarity). Interacts with MAPK8IP2; may regulate the MAPK8IP2 scaffolding activity (PubMed:11378392). Not secreted. Localizes to the lateral membrane and intercalated disks of myocytes. Additional isoforms seem to exist. Detected in brain, eye and heart. In brain, the different isoforms display different patterns of expression. Expressed in brain and heart (at protein level). Isoform 3 is highly expressed in cardiac myocytes while isoform 1 is the most abundant in brain. Expressed in the subplate of the embryonic cortex and the axonal tracts in the intermediate zone, and in axonal tracts of projection neurons, specifically in the corticothalamic tract and the corpus callosum (at protein level). Isoform 2 is transiently expressed in the neocortex and hippocampus from 17 dpc to P7 (at protein level). In embryonic brain, present in all divisions of the central and peripheral nervous system and it is at least 5 times more abundant than other FHFs. Detected in the subplate, ganglionic eminences, and proliferative zones of the cortical wall at 14 dpc. Detected in the cortical plate of the cerebral cortex, hippocampus, and striatum from 17 dpc to P14. Expression is markedly reduced in adult brain where it is most abundant in hippocampus. Also detected in developing kidney. Expressed in developing chandelier neurons. May be phosphorylated. Conditional knockout mice lacking fgf13 in the cerebral cortex or mice lacking fgf13 in most tissues display similar phenotypes of impaired spatial acquisition and memory. The cued memory and the capacity of novel object recognition are altered. They also display anxiety-related and reduced depression-like behaviors. This is associated with a disorganization of cortical structure and neural circuits. The laminar formation of the neocortex is delayed and the hippocampal development is also affected. Belongs to the heparin-binding growth factors family.