Cathepsin B has been implicated in progression of various human tumors. Overexpression of cathepsin B mRNA, increased cathepsin B staining and elevated cathepsin B activity have been found in different human cancers. These occur especially at the invasive edges of cancers, suggesting a role for cathepsin B in tumor invasion. In some tumors, mRNA expression and protein staining for cathepsin B correlate with clinical progression. Cathepsin B can facilitate tumor progression directly through degradation of components of the basement membrane and extracellular matrix. In vitro studies show that cathepsin B may exert its degradative effects intracellularly or extracellularly, depending on the cell type and location of cathepsin B. Cathepsin B can also facilitate tumor progression indirectly through activation of other latent proteases and/or degradation of protein inhibitors of other proteases. Thus cathepsin B may be an integral component of the proteolytic cascade linked to malignant progression of human tumors.