CTSB and metastatic melanoma: Differential analysis of cathepsin B surface expression performed in cells found above (non-invading) or below (invading) the Matrigel-covered filter, clearly demonstrated that invading cells (either from primary or metastatic melanomas) showed a significantly higher amount of cathepsin B at the cell surface (p < 0.01) (results obtained in two representative cell lines from primary and metastatic melanoma lesions are shown in Figure 4A).