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Publisher Summary This chapter covers the structural chemistry and the biological aspects of pappalysin-1 also known as Pregnancy-Associated Plasma Protein-A (PAPP-A). PAPP-A was discovered in 1972 as an antigen present in human pregnancy plasma with no known function or activity. Along with PAPP-A, three other (nonproteolytic) pregnancy-associated proteins were described and were named PAPP-B, -C and -D based on the positions of precipitate lines in immunodiffusion experiments. Subsequently, PAPP-A was described as a homotetramer of two disulfide-bound subunits. Although predicted from the amino acid sequence, the proteolytic activity of pappalysin-1 was discovered only when its homodimer was identified in medium conditioned by human fibroblasts. Pappalysin-1 proteolytic activity is effectively inhibited by EDTA or 1,10-phenanthroline. Pappalysin-1/proMBP polyclonal antibodies also effectively block pappalysin-1 activity. The pappalysin-1/proMBP heterotetramer is seen as an approximately 500 kDa band in SDS-PAGE. After reduction of disulfide bridges, the pappalysin-1 subunit of about 200 kDa is clearly visible but proMBP migrates as a smear of 50–90 kDa visible by western blotting, not Coomassie staining. The pappalysin-1 homodimer has an apparent molecular mass of 400 kDa.