Imagine a villain that corrupts those it touches. It can convert other individuals into twisted doppelgangers that eventually gather to form a destructive legion. This sounds like a character from any one of a hundred comic books, horror movies or science-fiction shows. But this antagonist is real, and it affects the brains of tens of thousands of people every year. It’s a mutant version of a protein called SOD1, and it causes amyotrophic lateral sclerosis (ALS), better known as Lou Gehrig’s disease.

All proteins are long chains of amino acids that fold up in acts of molecular origami. This has to happen with great precision – if the protein ends up with the wrong shape, it won’t work properly. Some