This phenotypic mutant was identified in an ENU mutagenesis screen. A complementation test between nmf5 and C3HeB/FeJ-Scn8amed-J revealed that nmf5 is a new allele of Scn8a. Molecular sequence analysis revealed that an A-to-T transversion mutation had occurred that is predicted to change the amino acid isoleucine 1392 to phenylalanine in the encoded protein. This conserved residue is within the S5-S6 pore loop of transmembrane domain 3.