Researchers have said that a single type of cell in the lining of the bladder is responsible for most cas-es of invasive bladder cancer. Philip Beachy, PhD, professor of biochemistry and of devel-opmental biology, said they’ve learned that, at an interme-diate stage during cancer progression, a single cancer stem cell and its progeny can quickly and completely replace the entire bladder lining. With their model in place, the research-ers then conducted two main experi-ments in the mice: In the first experi-ment, they looked to see what would happen in animals exposed to BBN when the sonic-hedgehog-expressing cells were marked with a distinctive fluorescent color. In the second, they used genetic techniques to selectively kill those same cells in animals prior to exposure with BBN. In the first case, they saw something startling: After just a few months of BBN exposure, nearly the entire lining of the bladder was labeled with the fluores-cent green marker that indicated the cells had arisen from the sonic-hedgehog-expressing basal stem cells. When transplanted into other mice, those labeled cells were able to give rise to bladder cancers, but cells not ex-pressing sonic hedgehog did not. In the second case, no tumors grew in the animals in which the stem cells had been selectively killed — al-though the bladder architecture became severely compro-mised in the absence of stem cells to regenerate cells lost during the normal course of bladder function. Next the researchers tackled the question of whether bladder can-cers arise as the result of genetic changes to one or more of these bladder stem cells. To do so, they used a genetically engineered mouse with cells that fluoresce green, but which can be triggered to randomly fluoresce one of three other colors: red, blue or yellow. Known as a “rainbow mouse,” the animal allows researchers to more precisely deter-mine the origin of groups of cells. If all cells in a tumor are red, for example, it is much more likely that they origi-nated from a single cell. Further studies showed that, surprisingly, none of the cells in the most advanced, invasive carcinomas in the BNN-treated animals expressed sonic hedgehog — despite the fact that only sonic-hedgehog-expressing cells are able to give rise to the earlier stages of bladder cancer.