MCAM, also known as melanoma cell adhesion molecule (CD146) or MUC18, originally identified as a biomarker of melanoma progression, is a transmembrane glycoprotein belonging to the immunoglobulin (Ig) superfamily. Structurally, it consists of five Ig domains, a transmembrane domain, and a cytoplasmic region. A soluble form of CD146 has also been reported. In normal adult tissue, CD146 is primarily expressed by vascular endothelium and smooth muscle. CD146 is a key cell adhesion protein in vascular endothelial cell activity and angiogenesis, and has been used as marker of circulating endothelium cells (CECs).