Pregnancy-associated plasma protein A (PAPP-A) is a high molecular weight protein. htPAPP-A is a protein complex which comprises of two PAPP-A subunits and two proform of eosinophil major basic protein. It exists in the blood of pregnant woman. dPAPP-A is a htPAPP-A dimmer which is composed of two PAPP-A subunits. htPAPP-A serves as a useful marker for the diagnosis of Down syndrome (DS).