Via EurekAlert!: "A constellation of different stem cell populations within our skin help it to cope with normal wear and tear. By constantly proliferating, the stem cells allow skin to replenish itself, allowing each cell to be replaced by a new one about once a month. But the normal cycle of division and death within one or more of these stem cell types can sometimes be derailed by genetic mishaps. Such events are believed to spawn carcinomas and other deadly skin cancers ... [researchers] have now unmasked a long sought stem cell origin of carcinoma and identified the genetic lesions occurring within these cells that spur them on to malignancy. ... increased activity of a powerful oncogene called Ras combined with overly exuberant activity of a protein called ΔNp63α, stimulates the population of skin stem cells that produce keratin 15 - one of many keratin proteins found in the skin - promoting carcinoma development. ... When cells in the skin or anywhere else in the body sense a potentially cancer-causing threat such as an activated oncogene like Ras - which speeds up cell division - the cells cope by slamming on the brakes to trigger a process called senescence. This is a tumor-suppressive mechanism that halts cell division while allowing cells to stay metabolically active. ... Unfortunately there are a number of genetic events that can bypass senescence and push a cell down the road towards cancer."