The glucose transporter is now identified but may have modifications or other subunits that control its activity. The kinetics and inhibitor binding studies are consistent with the carrier model with different degrees of asymmetry and a single binding site that varies in specificity depending on the conformation of the protein. The physical structure could actually be quite different from the usual diagrams (rocking bananas), however, and could function as a monomer or higher oligomer. The binding site, or filter, that gives specificity could be in the middle as usually depicted; alternatively it could be entirely on the cytoplasmic side, where the protein is trypsin sensitive, and hydrophobic helices could span the membrane forming a simple channel. Possible restrictions on structures in the membrane from the hydrophobic nature of transmembrane segments of membrane proteins (62) may favor a globular domain outside the membrane as the binding site. Such speculations will have to await more structural information about the transporter.