Signalling by the C-X-C chemokine receptor type 4 (CXCR4) promoted by its cognate ligand, C-X-C motif chemokine ligand 12 (CXCL12), also known as stromal-cell derived factor-1alpha, or SDF-1 alpha, plays a major critical role in cell chemotaxis and F-actin polymerisation, which initially control cell migration in various cancer types. The gene discussed is CXCR4; the disease is cancer.