A protein called p53 is exciting cancer researchers around the world. In its normal form, it helps to stop cells from becoming cancerous. It orders cells that are potentially cancerous, for instance because they have been damaged by radiation, to commit suicide. But in a mutant form, it not only fails to suppress cancer cells, it actually boosts their growth. Professor Pierre May and his wife Evelyne, who work at a French molecular cancer unit near Paris, are working on both these characteristics of p53 in the hope of finding a molecular key to a new kind of cancer treatment.