Using shRNA knockdown of endogenous FGF14 in cultured cerebellar Purkinje neurons and replacement with informative mutants that blocked binding of FGF14 to NaV CTDs and eliminated the FGF14 extended N-terminus, we found that FGF14, and especially the N-terminus of the FGF14b isoform exerts specific kinetic effects on cerebellar NaV channels that support repetitive firing and therefore provides new information on the pathophysiology of SCA27. Here, FGF14 is linked to spinocerebellar ataxia type 27.