1 in CRC cell¿CAFs attachment and its impact on liver metastasis. CAFs were obtained after xenotransplantation of Mc38 cells into EGFP-C57BL/6 mice. Attachment experiments with CRC cells and CAFs (with or without TGFß1 and the inhibitory peptide P17) were carried out, as well as in vivo liver metastasis assays. TGFß1 induced adhesion of CRC cells to CAFs, whereas exposure to P17 abrogated this effect. Co-injection of Mc38 cells with CAFs intrasplenically increased liver metastasis, as compared to injection of tumor cells alone. Pretreatment of Mc38 cells with TGFß1 enhanced the metastatic burden, in comparison to untreated Mc38 + CAFs. TGFß1-pretreated Mc38 cells co-metastatized with CAFs to the liver in a highly efficient way. Importantly, the metastatic burden was significantly reduced (p < 0.001) when P17 was administered in mice. The number of PCNA+ and CD-31+ cells was also reduced by P17 in these animals, indicating a decrease in proliferation and angiogenesis upon TGFß1 signaling blockade. Through microarray analysis, we identified potential TGFß1-regulated genes that may mediate cancer cell¿stroma interactions to increase metastasis. In conclusion, TGFß1 promotes co-travelling of CRC cells and CAFs to the liver to enhance metastasis. Our results strongly support the use of TGFß1 targeted drugs as a novel strategy to reduce liver metastasis in CRC patients.