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CD199, also known as CCR9, is a member of the G protein-coupled receptor （GPCR） family and functions as a chemokine receptor. It is a ~42 kDa seven-transmembrane protein primarily involved in T cell development in the thymus and gut-associated immune responses. CCR9 is expressed on T lymphocytes and is upregulated in response to retinoic acid, which is produced by dendritic cells, facilitating the migration of T cells to gut-associated tissues. Its specific ligand is CCL25 （TECK）, and the interaction between CCR9 and CCL25 plays a crucial role in the recruitment and maturation of thymocytes. Additionally, CCR9 is differentially expressed in T lymphocytes of the small intestine and colon, suggesting its involvement in specialized immune responses within the gastrointestinal tract.