CD146, also known as MCAM (melanoma cell adhesion molecule) or MUC18, is a heavily glycosylated transmembrane glycoprotein with more than 50% of the mass from carbohydrates. It is expressed on epithelial and endothelial cells, fibroblasts, multipotent mesenchymal stromal cells, activated T cells and activated keratinocytes, and on some cancer cells, especially melanoma. The presence of CD146 on circulating blood cells has been confined to the activated T cells rather than circulating endothelial cells. CD146 mediates heterophilic cell adhesion and regulates monocyte transendothelial migration.