The human uPAR cDNA encodes a polypeptide of 335 amino acids including a N-terminal 22-residue secretion signal peptide and a C-terminal segment (30 amino acids) removable with the attachment of a glycosyl phosphatidylinositol (GPI)-anchor [5]. The mature protein (283 residues) is highly glycosylated and composed of three similarly sized (about 90 residues each) homologous domains (here referred to as DI, DII and DIII) and belonging to the Ly-6/uPAR protein domain family [6]. The biochemical and structural aspects of uPAR have been extensively investigated and reviewed in detail [7], [8] and are summarized in Fig. 1.