The Economist looks at cancer stem cells: "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. ... Not all investigators support the cancer-stem-cell hypothesis, but the share who do so is growing rapidly. A mere five years ago, few research papers on the subject were presented at big academic meetings. This year there were hundreds at one such meeting alone. Moreover, data from clinical trials based on the hypothesis suggest that it has real value for patients. As a result, drug companies have taken notice and are trying to develop substances that will kill cancer stem cells. ... At the moment, scientists being scientists, few are willing to be anything other than cautious. They have seen too many past cures for cancer vanish in a puff of smoke. The proof needs to come from patients - preferably with them living longer. But if the stem-cell hypothesis is indeed shown to be correct, it will have the great virtue of unifying and simplifying the understanding of what cancer is. And that alone is reason for hope."