Cancers grow from stem cells. That discovery should translate into better treatment for tumours of all types.STEM cells have a controversial reputation, but in truth they are what makes human life possible. Each tissue in the body grows from a particular sort of stem cell. When it divides, one of its daughters remains a stem cell while the other eventually turns into whatever tissue its mother was designed to produce—be it blood, muscle, nerve or whatever. That is how healthy tissues are renewed, and it is now looking likely that it is how unhealthy tissues are renewed, too. Indeed, many researchers think that the underlying cause of cancer is the brakes coming off the regulatory system that stops normal stem cells from reproducing too much. For one of the most important medical discoveries made in recent years is that cancers, too, have stem cells and that these appear to be the source of the rest of the tumour.