Aminopeptidases catalyse the hydrolysis of a wide range of N-terminal amino acid residues from proteins and polypeptides. These enzymes have a broad substrate specificity and are widely distributed in bacteria, yeast, plant and animal tissues. Functions of aminopeptidases include protein maturation, protein degradation, hormone level regulation and cell cycle control. Since aminopeptidase activity has been reported on the surface of tumour cells, they are very likely key players in tumour growth and metastatic proliferation.