PAPP A was originally isolated from pregnancy serum as heterotetrameric protein complex with molecular weight approximately 500 kDa, 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. Reduced level of PAPP A in pregnancy blood in the first trimester is associated with fetal Down syndrome. PAPP A is considered as one of the best biochemical markers in early pregnancy used to screen for Down syndrome during the first trimester (Wald, N. J. et al).