Tumor expression of αvβ3 integrin promotes spontaneous metastasis to the spine but not primary tumor growth. Cells (1 × 104 in 20 μl of 50% Matrigel) were inoculated into the mammary gland of Balb/c mice (15 mice per group). (a) Primary tumor volumes were measured three times per week. (b) Primary tumor weights were recorded on the day of harvest (day 39). (c) In vitro proliferation of 66cl4pBabe and 66cl4beta3 cells. The tumor burden in the (d) spine and (f) lungs was measured by real-time quantitative PCR as described in Materials and methods. Each point represents one mouse, but not all points are distinguishable. The average metastatic burden for each group is shown with a line. *P = 0.01. (e) H&E of a 66cl4beta3 spontaneous femoral metastasis. A metastatic nodule (M) in proximity of a blood vessel (BV) is seen invading through the cortical bone (C). Scale bar = 100 μm.