CXCR4 and its ligand stromal cell-derived factor-1 (a.k.a. CXCL12) are essential for proper fetal development. CXCR4 is also the major coreceptor for T-tropic strains of human immunodeficiency virus 1 (HIV-1), and SDF-1 inhibits HIV-1 infection. Additionally, SDF-1 and CXCR4 mediate cancer cell migration and metastasis. The N terminal domain of most chemokine receptors is the ligand binding domain and so the N terminal domain of CXCR4 is the binding site for SDF-1.