Not all stem cells work for the Jedi, scientists have learned. Some work for the Dark Side and cause cancer. In a conceptual revolution in cancer biology, it now appears that rogue adult stem cells could be responsible for cancer's habit of recurring and metastasising. If we are able to eradicate the cancer stem cell, we will be able to cure patients," says Dr Jean-Pierre Issa, of the MD Anderson Cancer Center in Houston.

This novel approach began in the 1990s, when scientists found that some leukaemia cells were more dangerous than others. The challenge is to identify and isolate these cancer-causing cells. This would make it possible to create therapies which target stem-cell defining proteins and destroy the cancer at the source.

Time magazine says that the field of cancerous stem cells is burgeoning. Paradoxically this is happening in part because of President George Bush's opposition to the use of embryonic stem cells. Some Federal funding which might have gone into them is now being redirected into cancer studies. But, as with all stem cell research, therapies are not imminent. "Don't expect anything before five years," says Irving Weissman, of Stanford University, "but be angry if you don't see anything in 15 years."