The CXCR4 antagonist GST–NT21MP, a 21-mer synthetic peptide derived from the N-terminal extremity of the viral macrophage inflammatory protein II, also called vMIP-II (NT21MP sequence: Leu–Gly–Ala–Ser–Trp–His–Arg–Pro–Asp–Lys–Cys–Cys–Leu–Gly–tyr–Gln–Lys–Arg–Pro–Leu–Pro, residues 1–21), abrogates SDF-1-induced cell growth, breast cancer cells adhesion and migration, and delays pulmonary metastasis in vivo [131,132]. The gene discussed is CXCR4; the disease is breast cancer.