Female BALB/c mice (5–7 weeks old) were injected subcutaneously or through the tail vein with 4T1-luc2-M cells that were infected with a control lentivirus (Lenti-pGCsi or Lenti-pLKO.1) or a recombinant lentivirus expressing a miR-101 precursor (Lenti-pGCsi-miR-101) or shCXCR7 (Lenti-shCXCR7) (n = 5 mice/group). A. Tumor volumes of subcutaneous BrC cell implantation models. B. Tumor growth progression in the implanted mice, as determined by in vivo luciferase imaging of the xenografts on days 7, 14, 21, and 35 after subcutaneous injection. Upper panel: left hind, Lenti-pGCsi-miR-101; right hind, Lenti-pGCsi. Lower panel: left hind, Lenti-pLKO.1; right hind, Lenti-pLKO.1-shCXCR7. C. Representative images of tumors and quantification of tumor weights 6 weeks after subcutaneous xenografting. D. Tumor metastasis progression in the implanted mice, as determined by in vivo luciferase imaging of lung metastases on days 1, 4, and 10 after tail injection. E. Representative images of mouse lungs and quantification of microscopic nodules in the lungs of each group 8 weeks after tail vein injection. F. TUNEL assay showing the percentage of apoptotic cells in each group. (B–E) Representative images are shown. (A, C, E, F) Data represent the mean ± SD of three replicates. *P < 0.05 and **P < 0.01.