CJD, although rare, is the most common human prion diseases, a group of lethal transmissible neurodegenerative diseases, related to the conversion of a normal cellular protein, the prion protein (PrPC), into a misfolded form (PrPSc) which accumulates in neuronal cells leading to intracellular spongiform changes and neuronal loss (Prusiner, 1998). The gene discussed is PRNP; the disease is Creutzfeldt Jacob disease.