PAPP-A is a large heterottetrameric glycoprotein of approximately 500 kDa, which was first discovered in serum from pregnant women. The hetreotetramer consists of two PAPP-A subunits and two proMBP subunits. Low maternal serum levels of PAPP-A in first trimester biochemical screening is used as a marker of Down’s syndrome. a secreted metalloproteinase which cleaves insulin-like growth factor binding proteins (IGFBPs). It is thought to be involved in local proliferative processes such as wound healing and bone remodeling. Low plasma level of this protein has been suggested as a biochemical marker for pregnancies with aneuploid fetuses (fetuses with an abnormal number of chromosomes). For example, low PAPPA may be seen in prenatal screening for Down syndrome.