MCAM and breast cancer: The second novel aspect of the above conclusion is that the tumor suppressor role of METCAM/MUC18 in the human ovarian carcinoma cell lines [23,24] is also opposite to the previously established role of METCAM/MUC18 in other cancer cells in that it acts as a tumor promoter in both prostate cancer cells and breast cancer cells and as a metastasis promoter in human melanoma cells, prostate cancer and breast cancer [26,28,29,30,31,34].