The facilitative transport of glucose across the plasma membranes of mammalian cells is catalyzed by a family of glucose transport proteins (GLUT). Four glucose transport proteins and a fructose transport protein have been identified. These transport proteins have unique tissue distributions and biochemical properties under- lying specific physiologic functions. GLUT1, the first GLUT isoform identified, is expressed at highest levels in the endothelial of barrier tissues such as blood vessels and the blood-brain barrier. GLUT2, found predominantly in liver, intestine, kidney, and pancreatic beta-cells, is a low-affinity glucose transport protein that is part of the glucose sensor in pancreatic beta-cells and facilitates either glucose uptake or efflux from the liver depending on the nutritional state. GLUT3 is the glucose transporter responsible for maintaining an adequate glucose supply to neurons. GLUT4 is the insulin-regulated glucose transporter found in adipose tissues, heart muscles, and skeletal muscles that is responsible for insulin-regulated glucose disposal.