In the decades since prions were discovered by Stanley Prusiner, scientists have become comfortable with the idea that they cause disease. But how an infectious agent made solely of protein can produce varying strains has remained unclear.

In the March 18 Nature, two papers based on studies of prion infections of yeast report that the conformational differences in the infecting prions determine strain variation.

“This has been a major mystery of the whole prion field,” said Jonathan Weissman, professor at the University of California, San Francisco, and senior author of the first paper. “For example, people are used to there being different viral strains, because every time you get a new mutation, you get a new strain out. But if you have an infectious protein and protein is the only component, how could you have strain differences?” he told The Scientist.

To answer this question,...

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