Serpin B7 is a serine proteinase inhibitor of the ovalbumin like B clade of serpins. It was first discovered in a search for agents responsible for thrombopoiesis, and maturation of megakaryocytes. SerpinB7 was identified as a protein that increased acetylcholine esterase stimulation from bone barrow progenitor cells, and injection of SerpinB7 in mice induced a 40% increase in platelets compared to controls. Shortly thereafter, in a search for kidney specific genes, SerpinB7 was described as a protein specific to mesangial cells, and was named megsin to indicate the tissue specificity of the protein.