Membrane complement regulatory proteins (mCRPs) that are expressed on breast cancer cells, as well as normal cells provide exquisite protection from complement-mediated lysis [13,28-30], after inadvertent C4b or C3b deposition or after a marginal initiation of the classical pathway subsequent to sensitization of cancer cells by exogenous monoclonal antibodies or low affinity endogenous antibodies. This evidence concerns the gene C4B and breast cancer.