Gerstmann-Straussler-Scheinker disease (GSS) is a type of prion disease. Prion diseases are a group of conditions that affect the nervous system. The main feature of GSS is a progressive degeneration of the cerebellum (a part of the brain that controls coordination, balance, equilibrium and muscle tone), as well as different degrees of dementia. It is caused by changes in the PRNP gene and inheritance is autosomal dominant.