A protein is a chain of amino acids that folds, in a fraction of a second, into an intricate three-dimensional shape, and that shape determines what the protein does. Predicting the shape from the sequence alone was, for half a century, one of biology's most stubborn open problems, important enough to have a name, the protein folding problem, and hard enough to resist every attempt. In 2021 a team at DeepMind published a system called AlphaFold 2 that had, to a first approximation, solved it.