The first protein structures were determined by x-ray crystallography in 1957 by John C. Kendrew and Max F. Perutz. As a bioinorganic chemist, I was delighted that the structures were myoglobin and hemoglobin, both heme proteins with big, beautiful iron atoms. It must have been an extraordinary experience to stare at a physical model of the structures and see something that had previously only been imagined. Not long afterward, Christian B. Anfinsen Jr. proposed that the structure of a protein was thermodynamically stable. It seemed possible that the three-dimensional structure of a protein could be predicted based on the sequence of its amino acids. This “protein-folding problem,” as it came to be known, baffled scientists until this year, when the papers we’ve deemed the 2021 Breakthrough of the Year were published.