MCAM and melanoma: HuMETCAM/MUC18 plays an intriguing dual role in the progression of many epithelial tumors [15] as a tumor and metastasis promoter in breast cancer [13,16,17,18] and prostate cancer [19,20,21,22,23,24,25] and as a metastasis promoter in melanoma [26,27,28]; in contrast, it acted as a tumor and metastasis suppressor in ovarian cancer [14,29,30,31] and in one mouse melanoma cell line [32].