 In the practice of medicine, many fundamental biological pathways that require tight-on, off-control, such as inflammation and circulatory homeostasis, are regulated by serine proteinases, but we rarely consider the unique protease inhibitors that, in turn, regulate these proteases. The serpins are a family of proteins with a shared tertiary structure, whose members largely act as serine protease inhibitors, found in all forms of life, ranging from viruses, bacteria, anarchya to plants and animals. These proteins represent up to 2-10% of proteins in the human blood and are the third most common protein family. This article was authored by Marie-Christine Bautin, Margaret Geiger, William P. Sheffield and others.