Cells in damaged organs release S1P and C1P, which creates a chemotactic gradient for stem cells circulating in PB, such as multipotent stromal cells (MSCs), endothelial progenitor cells (EPCs), and very small embryonic-like stem cells (VSELs). For reasons of simplicity, other potential chemoattractants, such as SDF-1, are not shown in this scheme. Organ tissue damage may, for example, be the result of ischemia (e.g., stroke or heart infarct) or the result of hypoxia in a growing tumor. Therefore, C1P could, on the one hand, chemoattract circulating MSCs and EPCs for physiological process of regeneration and, on the other hand, may play an unwanted role in recruitment of these cells to an expanding tumor.