DPP4, also known as dipeptidyl peptidase-4, adenosine deaminase complexing protein 2, CD26, or DPPIV, is a 110-kDa cell surface glycoprotein. It functions as a serine protease and cleaves a diverse range of substrates, including growth factors, chemokines, and other signaling peptides. It is notably expressed in T lymphocytes and regulates T cell activation. It has been implicated as a tumor marker in cancers such as T cell lymphomas and thyroid cancer. Antibodies against DPP4 may be useful in investigating apoptosis, immune response, signal transduction, and cancer.