In addition to having a higher affinity for the migration surface and CXCR4, CXCL12-γ is present at lower levels within the tumor environment [77], and lower levels of CXCL12 correlate with increased metastasis in mouse models and worse prognosis in breast cancer [14, 15, 77, 80, 81]. This evidence concerns the gene CXCL12 and breast cancer.