Mind your metals Iron–sulfur clusters are important cofactors for proteins involved in metabolism and electron transfer but are also sometimes found in enzymes involved in transcription and replication of DNA. In vitro expression of such enzymes can result in faulty cluster assembly and confusion about the composition of the functional enzyme. Using a careful anoxic purification scheme, Maio et al. found that the severe acute respiratory syndrome coronavirus 2 RNA–dependent RNA polymerase contains two iron–sulfur clusters at two sites previously observed to bind zinc ions. Mutation of the ligating cysteine residues resulted in loss of polymerase activity. A less severe loss of activity was seen in the zinc-containing enzyme. Treatment with the nitroxide drug TEMPOL resulted in degradation of the clusters, enzyme inhibition, and inhibition of viral replication in cell culture. Science , abi5224, this issue p. 236