MSH: An autosomal dominant familial prion disease with a wide spectrum of clinical presentations including ATAXIA, spastic paraparesis, extrapyramidal signs, and DEMENTIA. Clinical onset is in the third to sixth decade of life and the mean duration of illness prior to death is five years. Several kindreds with variable clinical and pathologic features have been described. Pathologic features include cerebral prion protein amyloidosis, and spongiform or neurofibrillary degeneration. (From Brain Pathol 1998 Jul;8(3):499-513; Brain Pathol 1995 Jan;5(1):61-75),NCI: A very rare and fatal disorder of spongiform encephalopathy usually caused by mutations of the prion protein (PRNP) gene. It is characterized by the accumulation of amyloid in the brain. Signs and symptoms include lack of motor coordination, unsteady gait, and difficulty walking. As the disease progresses, patients develop speech difficulties and dementia.