CCL21 and breast cancer: Ccr7 (binds Ccl19 and Ccl21) and Cxcr4 (binds Cxcl12 (also named stromal cell-derived factor 1α)) have been associated with the organ specific metastatic spreading of breast cancer cells (for review see [39]), while syncytin A, syncytin B and its receptor Slc1A5 [25] have been linked to cell fusion