It now seems likely that medical science is on the brink of a finding of equal significance. The underlying biology of that scourge of modern humanity, cancer, looks as though it is about to yield its main secret. The discovery—or, rather, the hypothesis that is now being tested—is that cancers grow from stem cells in the way that healthy organs do.
A stem cell is one that, when it divides, produces two unequal daughters. One remains a stem cell while the other multiplies into the sorts of cells required by its organ. This matters for cancer because, at the moment, all the cells of a tumour are seen as more or less equivalent.
Therapies designed to kill them do not distinguish between them. Success is defined as eliminating as many of them as possible, so those therapies have been refined to do just that. However, if all that the therapies are doing is killing the descendants of the non-stem-cell daughters, the problem has not been eliminated. Instead of attacking the many, you have to attack the few. That means aiming at the stem cells themselves.