We describe a number of biochemical and pathological findings in a neurological syndrome which resulted from a mutation in the congenic resistant strain, B10.C(47N). The mutation, designated with the symbol msd, is sex-linked, recessive, and causes tremor, repeated seizures, and death of affected males at about 18–23 days of postnatal age. The earliest detectable clinical signs appear around 10 days of age. The CNS is markedly deficient in myelin. The white matter fiber tracts appear essentially as “negative images”. In contrast, the peripheral nerves are myelinated.