CST4 and neoplasm: Among the factors that promote tumor growth and invasion, several protease systems, which are involved in proteolytic degradation of extracellular matrix components, have been studied, including papain-like lysosomal cysteine proteases such as cathepsins B and L, as well as their endogenous inhibitors cystatins, for example cystatin A (also known as stefin A), cystatin B, and cystatin C.8 However, their roles in tumor progression are not clear yet.