CD46 is a type I membrane protein and is a regulatory part of the complement system. It has cofactor activity for inactivation (through cleavage) of complement components C3b and C4b by serum factor I, which protects the host cell from damage by complement. It may also be involved in the fusion of the spermatozoa with the oocyte during fertilization. Recently, CD46 has emerged as a promising target for the treatment of both adenocarcinoma and neuroendocrine types of metastatic castration-resistant prostate cancer (mCRPC).