Expression of this thyroid hormone transporter in mouse, but not human, brain endothelial cells is consistent with previous observations and explains the lack of central nervous system (CNS) hypothyroidism in mouse models of Allan-Herndon-Dudley Syndrome (AHDS) generated by Slc16a2 (MCT8) knockout6. The gene discussed is SLC16A2; the disease is Allan-Herndon-Dudley syndrome.