Plasminogen activator inhibitor-1 (PAI-1) belongs to the serpin family of serine proteinase inhibitors. Serpins inhibit their target proteinases by an ester bond being formed between the active site serine of the proteinase and the P1 residue of the reactive centre loop (RCL) of the serpin, fol-lowed by insertion of the RCL intob-sheet A of the serpin. Concomitantly, there are conformational changes in the flexible joint region lateral tob-sheet A.