CXCR4 and neoplasm: The highest tumor-to-blood (9.7 ± 2.2) and tumor-to-muscle (41.0 ± 9.7) ratios were observed at 96h p.i. Non-specific retention of radioactivity was observed in the liver and spleen at 25 and 8 %ID/g, respectively, at 96h p.i. These results were further confirmed by high CXCR4 immunoreactivity observed in excised H1155 tumors compared to A549 tumors (Figure 2G).