Matrix metalloproteinases (MMPs)1 constitute a family of zinc enzymes that are capable of cleaving a wide range of extracellular matrix proteins and soluble and cell surface-associated regulatory proteins (1). MT1-MMP, a prototypic member of a membrane-anchored MMP subfamily, is distinguished from soluble MMPs by the existence of a C-terminal transmembrane domain that associates the protease with the lipid membrane bilayer and a cytoplasmic tail that interacts with the intracellular milieu (2). Currently, six membrane-type MMPs are known: MT1-, MT2-, MT3-, MT4-, MT5-and MT6-MMP (1). In contrast to the soluble MMPs, MT1-MMP is ideally positioned for regulating pericellular proteolysis (3, 4). Our prior studies and the results of other investigations have revealed a key role for MT1-MMP in carcinogenesis and malignancy and demonstrated that MT1-MMP is a likely centerpiece of malignant transformation and that MT1-MMP contributes directly to tumor cell migration, invasion, and metastasis (5-9). Proteolysis of matrix alone, however, does not fully explain the important and unique role of MT1-MMP in tumor growth and malignant progression and, in particular, in metastasis.