In breast cancer cells, it was reported that, unlike CXCR4, the expression of CXCR7 was suppressed by estrogen receptor-mediated signaling [10, 15], leaving the question of how metastatic cancer cells up-regulate both CXCR4 and CXCR7 unanswered. This evidence concerns the gene ACKR3 and metastatic malignant neoplasm.