There are at least two potential explanations for this limitation: (i) the blood-brain barrier (BBB) may restrict access of radiolabeled peptides to CXCR4-expressing tumor cells and (ii) a considerable fraction of CXCR4 staining in gliomas is intracellular and intracellular CXCR4 may not be accessible to the peptides. This evidence concerns the gene CXCR4 and central nervous system cancer.