We report a 49-year-old woman with a history of progressive gait disturbance, white matter disease, and cerebrospinal fluid immunoglobulin abnormalities who met criteria for primary progressive multiple sclerosis and whose son died at age 10 years of an unknown congenital neurodevelopmental disorder. Sequencing of the proteolipid protein 1 gene showed a novel mutation, Leu30Arg (c.89TG), in the mother and son. Pelizaeus-Merzbacher disease is the cause of death in the son and explains the mother's adult-onset neurological disorder. This case goes against dogma that mothers of severely affected sons are asymptomatic as adults and expands the differential diagnosis of primary progressive multiple sclerosis to include proteolipid protein 1 gene mutations.