(A) Lungs from tumor-bearing NSG mice were extracted and analyzed for GFP expression by quantitative RT-PCR. GFP expression correlates with the number of K562-GFP tumor cells present. The level of lung metastasis was significantly lower in mice injected with siUCKL-1 than in sicontrol-injected mice (mean ± SD, n = 34, ** P ≤ 0.01). (B) At time of sacrifice, 500 µl blood was removed from the heart. RNA was extracted and analyzed for GFP by quantitative RT-PCR. Mice bearing siUCKL-1-injected tumors had significantly fewer circulating tumor cells than mice bearing sicontrol-injected tumors (mean ± SD, n = 13, * P ≤ 0.04). No GFP expression was detected in mice without tumor present.