Examples of structural inheritance include the propagation of prions, the infectious proteins of diseases such as scrapie (in sheep and goats), bovine spongiform encephalopathy ('mad cow disease') and Creutzfeldt–Jakob disease (although the protein-only hypothesis of prion transmission has been considered contentious until recently). Prions based on heritable protein structure also exist in yeast. Structural inheritance has also been seen in the orientation of cilia in protozoans such as "Paramecium" and "Tetrahymena", and 'handedness' of the spiral of the cell in "Tetrahymena", and shells of snails. Some organelles also have structural inheritance, such as the centriole, and the cell itself (defined by the plasma membrane) may also be an example of structural inheritance. To emphasize the difference of the molecular mechanism of structural inheritance from the canonical Watson-Crick base pairing mechanism of transmission of genetic information, the term 'Epigenetic templating' was introduced.