MCAM and breast cancer: Additionally, MCAM is abundant in metastatic melanoma and breast cancer cells of the worst grade; in these cells, MCAM readily recognizes and binds to extracellular S100A8/A9, which induces lung metastasis through the activation of the MCAM downstream pathways—i.e., the tumor progression locus 2 (TPL2)-ETS variant transcription factor 4 (ETV4)–matrix metallopeptidase 25 (MMP25) axis in the case of melanoma and the (TPL2)–(ETV4)–zinc finger E-box binding homeobox 1 (ZEB1) axis for breast cancer, both of which provide cancer cells with a metastatic driving force [13,14].