CD46 is one of the membrane complement regulatory proteins. CD46 possesses C3b-binding and factor I cofactor activities which play important roles in the regulation of the complement activation pathway. CD46 is widely distributed on blood cells, endothelial cells, epithelial cells and tumor cell lines. CD46 exists as many isoforms in a variety of tissues. The antigen has a broad distribution and is present on leukocytes, platelets, endothelial cells, epithelial cells and fibroblasts. It is strongly expressed on salivary gland ducts and kidney ducts, moderately on lymphocytes and endothelium, and weakly on interstitial tissues and muscle cells, but not on erythrocytes. It is expressed on thymocytes, B cells, monocytes, granulocytes, NK cells, platelets, epithelial and endothelial cells.