**4.3 Distribution of density variants**

Prion strain polymerization is a sequential process where single molecules are converted to polymers via a multitude of conformational variants. Different prion strains have been identified in animal and human cases based upon differential population densities of these quaternary structures [87]. Quaternary structure conformers of PrP have been isolated and studied using sucrose density gradient by many groups [88–91]. Differential prion strains have been also identified for the rapidly progressive forms of Alzheimer's disease with distinct population of highdensity PrP oligomeric species [92].
