Glucose Transporter 4 (GLUT4/SLC2A4) is a member of a family of proteins that facilitate glucose transport. In response to insulin GLUT4 translocates from low density microsomes to the plasma membrane. Activation of phosphatidylinositol-3-OH kinase is required for this trafficking event but is not sufficient. Insulin resistant subjects with and without diabetes show abnormal accumulation of GLUT4 in a dense membrane compartment in muscle and adipocytes from which insulin is unable to recruit GLUT4 to the cell surface.