CTSB and squamous cell carcinoma: The squamous cell carcinoma antigens (serpins B3 and B4) are tumor-associated proteins that can inhibit papain-like cysteine proteases, including cathepsins L, K and S. This is an example of glycosaminoglycan enhancement of B-clade serpin activity with heparin acting as a cofactor in serpin cross-class inhibition of cysteine proteases.