Aminopeptidase N is a receptor for human coronavirus (Yeager et al. 1992); it has an important role in HIV entry (Pulido-Cejudo et al. 1997) and in the processing of peptide for presentation by antigen-presenting cells (Hansen et al. 1993). This peptidase was shown to be involved in the degradation of extracellular matrix in tumor invasions. Its levels are elevated in cancer subjects and correlate with tumor load, and thus, the enzyme is believed to play a role in angiogenesis (Saiki et al., 1993). The catalytic domain of CD13 faces the exterior of the plasma membrane and is anchored by a transmembranespanning domain, which is composed of N-terminal residues 9-32 (Shipp and Look, 1993). An N-terminal truncated major isoform of CD13 lacking residues 1-58 has been purified from normal human serum (Watanabe et al., 1998), but also a minor isoform 43-truncated CD13 (Kawai et al., 2001). The biological significance of this proteolytic cleavage mechanism is yet obscure.