A prion disease (also called transmissible spongiform encephalopathy) is a disease which is caused by prions. Prions are structurally altered versions of small proteins that are normally expressed in cells. Unlike diseases that are caused by a mutation of the gene resulting in the expression of a mutant protein, prions are able to replicate and transmit diseases through physical contact with the normal proteins resulting in a structural change from the normal state to the prion state.