sFRP1 inhibits the metastatic potential of MDA-MB-231 tumor cells. MDA-MB-231/secreted Frizzled-related protein 1 (sFRP1)-P2 cells or control-P2 cells (1 × 106) were injected into five and six Balb/c nude mice, respectively, through the tail vein; 53 days later the mice were sacrificed and the lungs were dissected. (a) Representative picture of lungs from the mice injected with MDA-MB-231/sFRP1-P2 cells (left) and control-P2 cells (right); arrow, metastatic lesion. (b) The total number of lung surface metastases was determined for all the mice and the average is shown ± standard error of the mean. **P < 0.01.