A Pol β was knocked down in MCF-7 and A549 cells. Western blot analysis of protein levels of Pol β. B, D Representative images of transwell assay for cell migration (B) and invasion (C) in MCF-7 and A549 cells. The bottom part shows the relative percentage compared to negative control (**P < 0.01, by Student’s t test). D Actin stained with phalloidin (green) showed cell spreading in MCF-7 and A549 cells. E MCF-7 cells containing vector or Pol β shRNA were injected subcutaneously. Tumor growth were measured every three days (n = 10) (*P < 0.05, by Student’s t test). F Tumors were collected and stained with H&E to test tumor invasion. G Representative bioluminescent (BLI) images of SCID mice after intravenous injection with A549-vector and A549 knockdown Pol β cells (n = 5). The quantification of bioluminescence imaging signals was showed in right. (*P < 0.05, by Student’s t test).