Membrane cofactor protein isoform 14, also known as CD46, is a type I membrane protein that has cofactor activity for inactivation of complement components C3b and C4b by serum factor I. This function resides in the complement control protein repeats (CCPs), with CCP1–4 essential for regulation. CD46 is widely distributed on thymocytes, T cells, B cells, monocytes, granulocytes, NK cells, platelets, endothelial cells, epithelial cells, fibroblasts, placenta and sperm, but not on erythrocyte. Recombinant human CD46 protein, fused to His-tag at Nterminus, was expressed in E.coli and purified by using conventional chromatography techniques.