MMP-1 knockdown in BR3 cells attenuates lung metastasis. MMP-1 shRNA expressing 231-BR3 cells sh1 and control shCtr cells were injected into the tail veins of nude mice at the density of 2.5×105/100 μl PBS. Each group includes ten mice. Mice were sacrificed and lung sections were analyzed after 9 weeks. A, representative H&E staining of lung sections. Arrows indicate metastatic foci. B, for each group, total lung metastatic foci were counted. Fisher’s exact test showed a significant difference between the numbers of lung foci in the sh1 group compared to shCtr group (P < 0.05).