CXCR4 and cancer: Of particular interest, the CXCR4/CXCL12 interaction was the first ligand–receptor pair identified to play a critical role in determining the metastatic destination of breast cancer cells to the lungs (Müller et al, 2001) and has, since then, been involved in the metastatic spread of many other cancer cells to various target organs (Payne and Cornelius, 2002; Taichman et al, 2002; Smith et al, 2004; Perissinotto et al, 2005; Schimanski et al, 2005; Sun et al, 2005).