<Prions> are proteinaceous infection particles. They are probably devoid of nucleic acid. <Prion protein> (PrP) is encoded by the normal cellular gene. <Pathological isoform> of prion protein has different conformation than the normal PrP protein. The exact nature of the posttranslational modification of prion protein is unknown. This modification may be caused by infection with prions infectious disease or occures spontaneously sporadic disease. Some mutations in PrP gene results in the production of pathological protein familial disease.