CC chemokine receptor type 9, member of the class A family of seven-transmembrane G protein-coupled receptors
CCR9 is a homeostatic receptor specific for CCL25 (formerly known as thymus expressed chemokine) and is highly expressed on both immature and mature thymocytes as well as on intestinal homing T Lymphocytes and mucosal Lymphocytes. In cutaneous melanoma, activation of CCR9-CCL25 has been shown to stimulate metastasis to the small intestine. Chemokines are principal regulators for leukocyte trafficking, recruitment, and activation. Chemokine family membership is defined on the basis of sequence homology and on the presence of variations on a conserved cysteine motif, which allows the family to further divide into four subfamilies (CC, CXC, XC, and CX3C). Chemokines interact with seven-transmembrane receptors which are typically coupled to G protein for signaling. Currently, there are ten known receptors for CC chemokines, seven for CXC chemokines, and single receptors for the XC and CX3C chemokines. The CC chemokine receptors are all activating the G protein Gi.