Based on our studies indicating that brain metastatic CAF aggregates produced higher levels of chemokines CXCL12 and CXCL16 as compared to normal breast fibroblasts or primary tumor CAF aggregates, we performed cancer cell migration assays (using MCF-HER2 cells or patient-derived cancer cells) to investigate the relative propensity of breast cancer cells to migrate to these different microenvironments. This evidence concerns the gene ERBB2 and breast cancer.