Frederick Sanger, at the University of Cambridge, completes in 1953, after more than a decade of methodical work, the first complete determination of the exact amino acid sequence of a protein: bovine insulin. Until then it was known that proteins were composed of chains of twenty distinct amino acids, but it was unknown whether a specific, determined order existed for each protein or whether the sequence was in some way variable or random. Sanger develops his own chemical reagent — dinitrofluorobenzene, known as Sanger's reagent — capable of tagging the end of an amino acid chain, allowing him to methodically identify, fragment by fragment, the exact order of the 51 amino acids making up insulin's two chains. The result definitively demonstrates that each protein has a unique, fixed, genetically determined amino acid sequence — not random — establishing a foundational principle of modern biochemistry and opening the way to understanding how a protein's sequence determines its three-dimensional structure and biological function. Sanger would receive a second Nobel in 1980 for developing an equivalent method for sequencing DNA, becoming the only person with two Nobel Prizes in Chemistry.