Human CD199, also known as CCR9, is a member of the G protein coupled receptor family and is involved in T cell development in the thymus and in gut-associated immune response. Highly expressed on different stages of thymocytes and upregulated on CD4+CD8+ cells. It is expressed to a much lower level on spleen and lymph node cells. Expression of CCR9 on gamma/delta T cells in the intraepithelial and small intestine is reported. The interaction of CCR9 with its ligand CCL25 (TECK, thymus-expressed chemokine) may direct the trafficking of developing T cells in the thymus and generation of gut-specific immunological memory.