CXCR4 and neoplasm: Another key pro-tumorigenic signaling employed by many tumors is up-regulation of CXCR4, the receptor for chemokine CXCL12 (also known as SDF-1), and recruitment of stromal cells expressing CXCL12 which maintain the tumor and recruit additional cells to support tumor expansion and invasion, such as endothelial cell progenitors which contribute to neovascularization [64].