"Mad cow" research. Scientist in protective clothing, working through an isolator, handles a mouse infected with "mad cow" prions. This is a high-security level 4 laboratory for dangerous research. Prions are virus-like organisms which attack nerve cells. They cause Creutzfeldt-Jakob disease in humans; BSE (Bovine Spongiform Encepha- lopathy) or "mad cow" disease in cattle; and scrapie in sheep and goats. These brain diseases are fatal. Research on transgenic mice will help to determine whether prions are transmitted across species. It may answer the question whether eating "mad cows" causes human infection.