Tetrameric GLUT3, the SLC2A3 gene product, associated with the plasma membrane, mediates the facilitated diffusion of glucose (Glc) into cells. GLUT3 is expressed by many cell types, notably in the brain. Its low Km for glucose (~1 mM) relative to normal blood glucose concentration (~5 mM) allows these cells to import glucose independent of fluctuations in blood glucose levels (Colville et al. 1993). GLUT3, like GLUT1 and 4, has a high affinity for glucose.