CCR9 (C-C chemokine receptor type 9) belongs to the β-chemokine receptor family of the G-protein coupled receptor superfamily. Chemokine receptors play a crucial role in cell migration and are considered to be important therapeutic targets for inflammatory diseases and in tumor chemoresistance and metastasis. In addition, high expression of CCR9 has been reported in a wide range of cancers including melanoma, ovarian cancer, lung cancer, breast cancer and colon cancer. Thus, CCR9 is considered to be a potential tumor biomarker in diagnosis and therapy of cancer^1,2. Under physiological conditions, CCR9 is mainly expressed in immature T lymphocytes and on the surface of intestinal cells².

CCR9 receptor interacts with its specific ligand- chemokine ligand 25 (CCL25, thymus-expressed chemokine) in a two-step binding mechanism in which the C-terminal region of CCL25 first binds the N-terminus region and extracellular loops of the receptor followed by the binding of the N-terminus of the chemokine. The two-step biding mechanism stabilizes the receptor in an active conformation that facilitates intracellular signaling¹.