Iron makes up 35% of the Earth's mass, and is plentiful in its crust (approximately 5%), so it is not surprising that Biology has found many different applications for it. Iron–sulfur (Fe–S) clusters are essential, ubiquitous inorganic cofactors in electron-transport proteins of respiration and photosynthesis, and are responsible for the activity of hundreds of enzymes1. Various types of clusters (Figure 1) occur in iron-sulfur proteins, bound covalently to protein ligands, usually cysteine sulfur. Their activity is not confined to oxidation/reduction; in enzymes such as aconitase, they are involved in substrate binding and conversion. Fe–S enzymes that catalyse difficult reactions, such as nitrogenase in nitrogen fixation and hydrogenase in hydrogen production, contain complex ‘superclusters’2.