 Radioactively labelled amino acids have been used for years to delineate primary brain tumors and for the early detection of tumor recurrence. Positron emission tomography studies indicate that the extent of amino acid uptake correlates to the true histological extent of gliomas. Recently a fluorine-18 labelled amino acid has been introduced O-2-F-18Fluorethyl-L-tyrosin FET, which is suitable for routine use in brain tumor patients. There is evidence that this amino acid is transported into brain and brain tumors by the amino acid transport of the L-type. The cDNA of this L-transporter has recently been cloned and has been shown to be identical to the light chain of the 4F2-antigen CD98, which has previously been described as marker of cell growth and proliferation. 