Though CXCR4 affinity was low for these compounds, the radiotracers [18F]RPS-534 (26, RPS-534, IC50 218 ± 38 nM) and [18F]RPS-547 (27 ̧ RPS-547, IC50 601 ±118 nM) displayed good tumor-to-blood, tumor-to-muscle, and tumor-to-lung uptake ratios in PC3-CXCR4 xenograft tumors (Figure 12). The gene discussed is CXCR4; the disease is neoplasm.