Binding of the chemokine CXCL12 to its receptor CXCR4 stimulates downstream G protein signaling, leading to the activation of the PI3K/Akt or MAPK pathway, and the CXCL12/CXCR4 axis is a key step in breast cancer cell migration toward the lungs [73]. Here, CXCR4 is linked to breast cancer.