Prion diseases or transmissible spongiform encephalopathies (TSEs) are a family of rare progressive neurodegenerative diseases that affect both humans and animals. They are characterized by long incubation periods, characteristic spongiform changes associated with neuronal loss, and an inability to induce an inflammatory response. The infectious protein particles, the prion, were identified by the American neurologist Stanley B. Prusiner and colleagues in 1982, deriving from the words protein and infection (Stanley B. Prusiner -Autobiography). Prions are known to occur in Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia in humans. In mammals they cause "mad cow disease" in cattle. Prion diseases affect the structure of the brain or other neural tissues and are all currently universally fatal and incurable (Prusiner, 1998). Transmissible spongiform encephalopathies (TSEs) are caused by abnormal folding of prions. The normal prion protein, called PrPc, is a 35 kD, water-soluble, proteinase-sensitive membrane glycoprotein (Laurén, 2009). Abnormal prions, designated PrPSc or PrPTSE, result from a change in the protein's folding pattern. PrPc, which makes it resistant to the action of proteases and causes it to precipitate as insoluble amyloid (Robbins, 1999). This conversion causes neuronal degeneration and loss by an unknown mechanism. PrPc is encoded by the PRNP gene on chromosome 20 and likely plays a role in multiple cellular functions, including cell adhesion, ion channel activity, and neuronal excitability ( Lindquist, 2001 ). The structure of the PrPC protein is composed of a normal protein present on the cell membrane. PrP has been reported to play a significant role in intr...... middle of paper ......, suggesting that direct proximity of cells promotes efficient infection. The fact that living cells are much more effective at transmitting infectivity than dead cells suggests that cellular biological processes are involved in prion transmission (MacKenzie, 2011). Potential treatments and diagnoses Biologics represent an ever-increasing fraction of all therapies, but all eukaryotically produced biologic bacteria carry some risk of prion contamination, even when generated in cell lines (Brown, 2003). The transmission of CJD through blood and even purified blood products has dramatically highlighted the severity of this threat. Therefore, PrPC-deficient farm animals (cattle and goats) are well positioned for the production of prion-free drugs and will therefore make an important contribution to eliminating the risk of prion contamination in biological processes.