(GSS) is a very rare, usually familial, fatal neurodegenerative disease that affects patients from 20 to 60 years of age. Similarly to Creutzfeldt–Jakob disease (CJD), GSS is a transmissible spongiform encephalopathy caused by misfolding and aggregation of the prion protein. The exact incidence of GSS is unknown but is estimated to be between 1 to 10 per 100 million. Unlike CJD, which can be sporadic or acquired, as in the case of vCJD (see above), GSS is inherited and is found in only a few families in the world. Symptoms typically start with difficulty speaking and ataxia and then progress to dementia.