The human serpin superfamily consists of at least 35 members that target not only serine proteases, but also selected cysteine proteases and non-protease proteins. Serpins bind the protease active site resulting in a major conformational rearrangement that traps the enzyme in a covalent acyl-enzyme intermediate. As protease inhibitors, serpins have an array of functions including regulating blood clotting, the complement pathway, extracellular matrix remodeling, and cell motility.