CXCR4 and melanoma: For example, CXCR4 (CXC chemokine receptor-4) is expressed on the surface of melanoma cells, while its ligand CXCL12 is released by MAFs in the tumor microenvironment, promoting the migration and metastasis of melanoma cells to distal metastatic sites through interaction with CXCR4 expressed on tumor cells (277–279).