Arsenic is a carcinogen associated with increased risk of skin, kidney, lung, and bladder cancer. The arsenic toxicity led to the evolution of arsenic defense mechanisms in every organism, from Escherichia coli to man. A putative arsenate reductase from Yersinia pestis shows 47.9% amino-acid identity to Escherichia coli arsenate reductase ArsC. ArsC is a part of bacterial system for the detoxification of arsenate, arsenite, and antimonite. Bacterial detoxification systems can transport arsenite and antimonite out of the cell. Arsenate, however, must first be reduced to arsenite by arsenate reductase before it is excreted. ArsC uses reduced glutathione (GSH) and glutaredoxin as source of reducing potential to convert arsenate to arsenite. Some arsenate reductases use thioredoxin instead of glutaredoxin in their reactions.