In metastatic colorectal cancers, tumor cells are disseminated prior to surgical resection of the primary tumor but remain dormant until proper colonization mechanisms are activated. To identify the colonization mechanisms of the metastatic tumors, we conducted a pairwise comparison between primary colorectal cancers and metastatic tumors (n=12 pairs), including six hepatic pairs and six pulmonary pairs. The mRNA levels of 224 genes previously reported to be associated with metastasis, cytokines and angiogenesis were quantitatively determined by PCR arrays. Among them, 27 genes were duplicated or triplicated to show consistent expression. Unsupervised hierarchical clustering of the Ct values of metastasis-related genes revealed that liver metastases were indistinguishable from primary colorectal cancers (n=5/6), whereas lung metastases were highly diversified from one another and from the primary tumors (n=6/6). Cytokines and receptor gene expression array data also confirmed the divergence of pulmonary metastases from primary colorectal cancers (n=6/6). Heat map analyses of ΔCt values of the metastasis-related genes identified a 17-gene tropism signature that was sufficient not only to distinguish liver and the lung metastases, but also reconstituted the clustering of primary tumors with the hepatic metastases (n=17/18). In this pilot experiment, pulmonary metastases were significantly diverged from hepatic metastases that were indistinguishable from primary colorectal cancers. Further genomic and clinical studies are in progress to evaluate the potential of the tropism signature as a therapeutic target to inhibit the colonization of metastatic colorectal cancers.