CD46, also known as Membrane Cofactor Protein (MCP), is a complement regulatory protein. CD46 is a type 1 membrane protein that plays an important inhibitory role in the complement system. CD46 is expressed in white blood cells, platelets, epithelial cells, and fibroblasts. Human CD46 shares 5% amino acid sequence identity with mouse and rat CD46. The importance of CD46 to complement regulation is underscored by the observation that genetic loss of CD46 leads to development of atypical hemolytic-uremic syndrome (aHUS), a disease characterized by uncontrolled complement activation. CD46 is implicated in the development and/or progression of selected cancer types.