PAPP A (PAPP-A, Insulin-like growth factor binding protein-4 protease, PAPP-A-proMBP complex) was originally isolated from pregnancy serum as heterotetrameric protein complex with molecular weight approximately 500kD, consisting of two PAPP A subunits disulfide-linked with two subunits of the proform of eosinophil major basic protein (proMBP). In such a complex proMBP functions as an inhibitor of PAPP A proteinase activity. PAPP A is widely used as a marker of some pathologies during pregnancy. Low maternal serum levels of PAPP-A in first trimester biochemical screening is used as a marker of Down’s syndrome (Trisomy 21).