This shows that the paracrine IL-6 effect on the migration/invasion of breast cancer cells is stronger than that of SDF-1, and that the pro-migratory/-invasiveness effects of CHEK2-deficient stromal fibroblasts is mediated through increase in the secretion of SDF-1 and IL-6, which are in normal situations repressed by CHEK2. Here, IL6 is linked to breast cancer.