In a previous study, we reported that a peptide antagonist of CXCR4, NT21MP (LGASWHRPDKCCLGYQKRPLP), derived from the residues 1–21 of viral macrophage inflammatory protein II efficiently inhibits SDF-1α-induced proliferation and invasion in breast cancer cells by reducing the levels of phosphorylated AKT and ERK1/2 [21–23]. This evidence concerns the gene MAPK3 and breast cancer.