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, which is an E clade serpin.

SerpinB2 is made by many cell types, and is found intracellularly as an unglycosylated kDa protein, and secreted as a 60 kDa protein. SerpinB2 is found in saliva, secreted by gingival fibroblasts, and in the skin. SerpinB2 levels are elevated in serum during pregnancy, and in leukemia, breast cancer and ovarian cancer, although it was lowered in some cancers.A shorter SerpinB2 isoform of 382 amino acids, has been reported, with a predicted mass of 43.1 kDa and a pI of 5.69. The shorter form has a deletion just after the start of the mature protein, but it is unclear what the relative production and distribution is for the shorter sequence.