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The classification of the prions is quite unique, and even being able to term them as organisms is questionable.  Because a prion is simply a conformationally altered protein, it cannot be classified according to traditional methods involving domain, kingdom, phylum, class, order, family, genus, and species. 


To provide clarity, a classification of the prion according to the mammals it infects is indicated.  Below is a phylogenetic tree showing similarities among mammal gene sequences affected by TSEs.  The numbers at the internal nodes are Bootstrap values greater than 50.

A phylogenetic tree I created.