Ribonucleotide reductase is the only enzyme that catalyses de novo formation of deoxyribonucleotides and is thus a key enzyme in DNA synthesis. The radical-based reaction involves five cysteines. Two redox-active cysteines are located at adjacent antiparallel strands in a new type of ten-stranded alpha/beta-barrel, and two others at the carboxyl end in a flexible arm. The fifth cysteine, in a loop in the centre of the barrel, is positioned to initiate the radical reaction.