The biological function of ketone bodies as a fuel assumes paramount importance in the brain (and retina) because neurons can use only glucose or ketone bodies for energy; therefore, when glucose availability is limited, as in prolonged starvation or genetic diseases associated with defective transfer of glucose into the brain (e.g., mutations in the glucose transporter GLUT1/SLC2A1), ketone bodies in circulation become the only other energy source for the brain (and retina). The gene discussed is SLC2A1; the disease is hereditary disease.