T cell maturation requires the controlled migration of T cell precursors through the different thymic compartments in a process that seems to be regulated by chemokines. The former human putative chemokine receptor GPR-9-6 has recently been identified as CCR9, the specific receptor for the thymus-expressed 0 chemokine TECK. The cDNA sequences containing thecomplete coding regionsofhuman and mouse CCR9 have been established, showing that the identity between both CCR9 predicted proteins is 86%. The expression of both genes is also very similar, being high in the thymus and low in lymph nodes and spleen. RT-PCR analysis of mouse CCR9 expression on murine FACS-sorled thymocyte subpopulalions showed that this gene is expressed in both immature and mature T cells. Among the 36 different chemokines tested on inlracyloplasmic calcium mobilization and in vitro chemotaxis assays. TECK is the only functional ligand found for CCR9. The expression of murine CCR9 in both immature and mature thymocytes, together with theexpression of its ligand in the thymus, suggests that the TECK-medialed chemoattraclion minhl be a mechanism contributing to thymocyte retention in the thymus while completing their development.