The exact molecular basis of the tumor-tropism of the F3.C1 line or other stem/progenitor cells, is not well understood, but factors such as stromal cell-derived factor-1 (SDF-1; CXCL12), scatter factor (SCF; HGF), vascular endothelial growth factor (VEGF) and macrophage chemotactic protein-1 (MCP-1) expressed by tumor cells likely play a chemotactic role [15], [36]–[42]. The gene discussed is CCL2; the disease is neoplasm.