A fake beauty spot could one day warn you of cancer. The implant, made of genetically engineered cells, has been designed to detect developing breast, prostate and colon cancers when they are only a few millimetres in size.

Martin Fussenegger of the Swiss Federal Institute of Technology in Zurich and his colleagues made this implant by genetically altering human skin cells so that they would become darker in colour when exposed to rising calcium levels. A rise in calcium in the blood is often the first …