Genetically engineered cells implanted in mice have cleared away toxic plaques associated with Alzheimer's disease. The animals were sickened with a human gene that caused them to develop, at an accelerated rate, the disease that robs millions of elderly people of their memories. After receiving the doctored cells, the brain-muddling plaques melted away. If this works in humans, old age could be a much happier time of life. The first published report of the experiments, done by Selkoe and other