GLUT1, the major glucose transporter in peripheral T lymphocytes, is induced upon T cell receptor activation. However, the role of GLUT1 during human thymocyte differentiation remains to be evaluated. Our identification of GLUT1 as the human T lymphotrophic virus (HTLV) receptor has enabled us to use tagged HTLVreceptor-binding domain fusion proteins to specifically monitor surface GLUT1 expression. Here, we identify a unique subset of CD4 ؉ CD8 ؉ double-positive (DP) thymocytes, based on their GLUT1 surface expression. Whereas these cells express variable levels of CD8, they express uniformly high levels of CD4. Glucose uptake was 7-fold higher in CD4 hi DP thymocytes than in CD4 lo DP thymocytes (P ؍ 0.0002). Further analyses indicated that these GLUT1 ؉ thymocytes are early post--selection, as demonstrated by low levels of T cell receptor (TCR)␣ and CD3. This population of immature GLUT1 ؉ DP cells is rapidly cycling and can be further distinguished by specific expression of the transferrin receptor. Importantly, the CXCR4 chemokine receptor is expressed at 15-fold higher levels on GLUT1 ؉ DP thymocytes, as compared with the DP GLUT1 ؊ subset, and the former cells show enhanced chemotaxis to the CXCR4 ligand CXCL12. Thus, during human thymopoiesis, GLUT1 is up-regulated after -selection, and these immature DP cells constitute a population with distinct metabolic and chemotactic properties.GLUT-1 ͉ metabolism ͉ thymus G lucose provides a key supply of energy and carbon for all living organisms, and its transport is a universally conserved property. Vertebrate glucose transporters belong to the GLUT family, of which GLUT1, first identified in 1985 by Mueckler and colleagues (1), appears to be the main functional glucosetransporter isoform in T lymphocytes (2). Interestingly, however, GLUT1 does not appear to be expressed on either quiescent human T cells (2) or their murine counterparts (3, 4). Rather, GLUT1 expression is induced upon T cell receptor (TCR) activation, a process long recognized as being associated with increased glucose metabolism (5-8). We recently determined that GLUT1 is, indeed, an early marker of human T cell activation, with surface expression detected as early as 4 h after TCR stimulation (9).In this context, glucose metabolism, in general, and GLUT1 expression, specifically, would be expected to be crucial to the survival, differentiation, and proliferation of developing T cells in the thymus. Indeed, Ͼ25 years ago, Whitesell and Regen (10) reported that the thymus contains two populations of cells: quiescent cells, where glucose transport equilibrates with a half-time of 30-50 min, and active cells, where the half-time is Ϸ1 min. However, studies attempting to correlate GLUT1 expression with glucose transport in the thymus were not undertaken, in large part, because specific GLUT1 detection has been hampered by a lack of reliable reagents recognizing exofacial GLUT1 determinants. Recently, however, Singer and colleagues (11) reported a large heterogeneity in GLUT1 expression in mur...