Serum markers are extensively used in diagnostic and follow-up of cancer patients. We recently described Nectin-4, a 66-kDa adhesion molecule of the Nectin family, which is a valuable new histological and serological marker for breast carcinoma. In vivo, Nectin-4 is re-expressed in breast carcinoma, and a circulating form of Nectin-4 is detected in the sera of patients with metastatic breast cancer. In vitro, a soluble form of Nectin-4 is produced in the supernatant of breast tumor cell lines (S. Fabre-Lafay, C. Ginestier, S. Garrido-Urbani, C. Berruyer, R. Sauvan, N. Reymond, J. Adé laide, J. Geneix, P. Dubreuil, J. Jacquemier, D. Birnbaum, and M. Lopez, manuscript in preparation). We have investigated the mechanisms that regulate the production of this soluble form. It was found that the soluble form of Nectin-4 detected in the sera of patients and the supernatant of breast tumor cell lines share similar biochemical and immunological features. The soluble Nectin-4 form (43 kDa) is formed by the entire Nectin-4 ectodomain. Nectin-4 shedding is constitutive, strongly enhanced by 12-O-tetradecanoylphorbol-13-acetate activation, and reduced tumor necrosis factor-␣ protease inhibitor TAPI-1 or by the tissue inhibitor of metalloproteinase-3 (TIMP-3). TAPI-1 and TIMP-3 are inhibitors of the endoprotease tumor necrosis factor-␣-converting enzyme (TACE)/ADAM-17. Overexpression or small interfering RNA-mediated silencing of TACE enhanced or reduced Nectin-4 shedding, respectively. Nectin-4 is not shed when expressed in TACE-deficient fibroblasts. Interestingly, the active form of TACE is overexpressed in breast tumors and may indicate that TACE is responsible for Nectin-4 shedding not only in vitro but also in vivo.The Nectin family comprises five transmembrane glycoproteins (PVR/CD155, Nectin-1/CD111, Nectin-2/CD112, Nectin-3, and Nectin-4), all members of the immunoglobulin superfamily (1-7). Nectins are both homophilic and heterophilic cell adhesion molecules (5,6,8,9). Nectin-2 and PVR interact with Nectin-3 and DNAM-1/CD226 (5, 10), PVR interacts with Tactile/CD96 (11), and Nectin-1 interacts with Nectin-3 and Nectin-4 (5). Nectins are widely expressed in adult tissues except for Nectin-4, whose expression pattern is restricted to the embryo (5). Nectin-4 is re-expressed in breast carcinoma and belongs to the class of embryonic carcino-antigens. It is a new valuable serum marker for breast carcinoma.1 Indeed, a soluble form of Nectin-4 is produced in the supernatant of breast tumor cell lines, and a circulating form of Nectin-4 is detected in the sera of patients with metastatic breast carcinoma.Cancer serum markers result from cell surface shedding from the tumor from which they originate. Ectodomain shedding is a major process regulating various biological events such as cell differentiation, proliferation, migration, and survival. Various proteins such as growth factors, growth factor receptors, or cell adhesion molecules are substrates for sheddases. Among the Zn 2ϩ -dependent protease superfamily, matrix metal...