Membrane Cofactor Protein (CD46) is a type 1 membrane protein that plays an important inhibitory role in the complement system. CD46 regulates the complement cascade by inhibiting C3b and C4b deposited on self tissue. This protein is a cofactor that binds to C3b and C4b, allowing their degradation by a plasma serine protease called factor I. This function resides in the complement control protein repeats (CCPs), with CCP1-4 essential for regulation. CD46 is implicated in the development and/or progression of selected cancer types.