Stromal-cell-derived factor-1-α is constitutively expressed in most tissues (Shirozu et al, 1995), whereas CXCR4 is expressed in many cancer cell lines and tissues (Burger et al, 1999; Muller et al, 2001; Bertolini et al, 2002; Schrader et al, 2002) and is able to mediate the metastasis of a number of cancers (Zlotnik, 2006). The gene discussed is CXCL12; the disease is cancer.