This Scn8a^med remutation occurred at The Jackson Laboratory on the same chromosome as the very closely linked gene Krt71 in a stock carrying a mutation in that gene (J:6191). Homozygotes resemble Scn8a^med homozygotes. Heterozygotes may show mild manifestations of the disease during the first 2 weeks but they recover. Both homozygotes and heterozygotes exhibit immunological aberrations: reduced PFC response to sheep red blood cells in 14- to 16-day old mice, with reduced suppressor cell function and precocious maturation of the cytotoxic response to allogeneic cells at 21 to 23 days. The reduction in the PFC response disappears in older heterozygotes but remains in the few homozygotes that survive beyond 6 weeks of age (J:6824). The molecular defect in Scn8a is a 4 bp deletion in the splice donor site of exon 3 that causes exon skipping and protein truncation (J:34154). A very low level of correctly spliced transcripts has been detected, indicating that Scn8a^med-J is a severe hypomorph (J:53340). The modifier locus Scnm1 on chromosome 3 influences the phenotype of homozygotes for the Scn8a^med-J allele (J:53340). C57BL/6J mice carry the susceptible allele of Scnm1 which results in juvenile death. Other inbred strains carry a resistant allele and on these strains, homozygotes exhibit muscle weakness and dystonia.