The CXCR4 (C-X-C chemokine receptor type 4) in breast cancer cells can promote metastasis by binding to ligands on the surface of hepatocytes or by mediation of tumor-associated macrophages, while HER2 can upregulate CXCR4 expression, which further promotes liver metastasis through the CXCL12/CXCR4 pathway (34). Here, CXCR4 is linked to breast cancer.