By injecting tumor cell/fibroblast cell suspension into immunodeficent mice, Orimo et al. found that CAF isolated from a human breast carcinoma and expressing a myofibroblast phenotype, promote the growth of carcinoma cells through a stromal cell-derived factor-1 [SDF-1]/CXCR4-dependent mechanism. The gene discussed is CXCL12; the disease is carcinoma.