ARSENIC is a carcinogen that packs a double punch: cells that it has made cancerous send out signals that in turn make healthy stem cells become malignant.

Michael Waalkes and his colleagues at the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina, had noticed that tumours triggered by arsenic contained unusually large numbers of cancer stem cells.

To find out why, the researchers turned cells from the human prostate gland cancerous with arsenic, before growing them in close proximity to, but not touching, healthy stem cells from the prostate. Even though these cells hadn’t themselves been exposed to arsenic, within three weeks they had turned into cancer stem cells (Environmental Health Perspectives, DOI: 10.1289/ehp.1204987).

Waalkes suspects a signalling molecule called interleukin-6 may be involved. “But I would not want to say it is the sole factor.”