CXCR4 and neoplasm: There are at least two potential explanations for this limitation: (i) the blood-brain barrier (BBB) may restrict access of radiolabeled peptides to many CXCR4-expressing tumor cells and (ii) a considerable fraction of CXCR4 staining is intracellular and intracellular CXCR4 may not be accessible to the peptides.