SerpinB2 is a serine proteinase inhibitor of the ovalbumin like B clade of serpins. It was first discovered in the placenta, and given the name PAI-2 because of the ability to inhibit urokinase plasminogen activator (u-PA) at low micromolar efficiency. SerpinB2 also inhibits tissue plasminogen activator (t-PA), but with micromolar efficiency, and PAI-1 is much more efficient that SerpinB2 on both forms of plasminogen activator. The structure of PAI-2 is not terribly similar to PAI-1, however,