An example of a class of stem cell medicine that involves manipulation of existing populations of cells in the body: "Circulating through the bloodstream of every human being is a rare and powerful type of cell, one that can actually create new blood vessels to bypass blockages that cause heart attacks and peripheral artery disease. Though everyone has these cells - called endothelial progenitor cells - they are often dysfunctional in people prone to vascular disease. Now researchers [have] discovered that a molecule - called Wnt1 - can improve the function of endothelial progenitor cells, increasing the blood flow to organs that previously had been cut off from the circulation. The finding could enhance clinical trials already testing these powerful cells in patients hospitalized with cardiac arrest. ... A number of studies in the past few years have suggested that genes that play an important role during early development and get 'turned off' during adulthood may also get 'turned on' or expressed again in response to injury, such as heart attack. [Researchers] found that one gene in particular, Wnt1, was expressed during development of blood vessels, shut off during adulthood and then re-expressed in angiosarcoma, a cancer of endothelial cells. ... treating these special cells with Wnt1 not only greatly increased their function but also their number. Next, [researchers] investigated what effect the protein would have on a mouse model of peripheral artery disease, an illness in humans caused by decreased blood flow to the extremities. They found that treating these animals with a single injection of the Wnt1 protein resulted in almost three fold increase in blood flow in the affected areas."