Endothelial cells were induced to express either GFP (green), RFP (red), YFP (yellow), or CFP (not shown), giving these cells a genetic label that allows tracing of daughter cells. Depicted is a section of an ischemic mouse skeletal muscle after injury. It shows a majority of the endothelial cells are red, indicating that the cells likely stem from a single parent cell. This phenomenon is termed clonal expansion and can contribute up to 69% of new vessels. See related article, page 670.