Urokinase Plasminogen Acitivator is a serine protease (SERPIN) that converts plasminogen into the active protease plasmin. uPA is secreted from cells as an inactive ~55kD precursor which is activated to the two-chain form consisting of an A and B-chain. The active high MW form is further processed by removal of an amino-terminal fragment to an active low MW form (~35kD). uPA is a serine protease that activates plasminogen to plasmin. uPA is involved in the degradation of the vascular basement membrane and extracellular matrix. By catalyzing plasmin formation, the uPA can generate widespread extracellular proteolysis and thereby play an important role in physiological and pathological processes. High levels of uPA and plasminogen activator inhibitor type 1 (PAI-1) in breast cancer tissue extracts have been associated with rapid disease progression. The malignant phenotype of prostatic tumor cells correlates with the expression of both uPA and its cell-membrane receptor (uPAR).