To investigate whether the E200K mutation is enough to de novo confer PrP(Sc) properties to mutant PrP, as suggested by experiments in Chinese hamster ovary cells, we examined the biochemical behavior of E200KPrP in brains and fibroblasts from sporadic as well as homozygous and heterozygous E200KCJD patients, asymptomatic transgenic mice carrying the E200K mutation, as well as in normal and scrapie-infected mouse neuroblastoma cells expressing E200KPrP.