CCR9 (C-C motif chemokine receptor 9) is a G protein-coupled chemokine receptor that plays critical roles in immune cell trafficking and T cell development. CCR9 binds to its primary ligand CCL25 and mediates the recruitment of specific T cell subsets to mucosal tissues, particularly the small intestine 1. During T cell development, CCR9 guides thymus seeding progenitors to migrate to the thymus alongside CCR7, facilitating T cell maturation 2. In cancer immunotherapy, CCR9+CXCR3+CD4+ T lymphocytes are recruited to tumor beds and correlate with improved responses to PD-1 blockade therapy 3. CCR9 also regulates the migration of memory-like group 2 innate lymphoid cells (ml-ILC2s) from intestinal lamina propria to airways during asthma relapse through CCR9/CCL25 signaling 4. Clinically, CCR9 represents a promising therapeutic target, as it is expressed on >70% of T cell acute lymphoblastic leukemia (T-ALL) patients and maintained at relapse, making it suitable for CAR-T cell therapy 5. Additionally, CCR9+ B cells are enriched in IgA nephropathy patients and preferentially home to mucosal sites 6. Overall, CCR9 functions as a key regulator of immune cell migration between systemic circulation and mucosal tissues.