Such altered expression patterns have been detected for cytokeratin 18, tissue transgluta-minase, Rho GDP-dissociation inhibitor 1, fibroblast-type tropo-myosin, interleukin-18, annexin I, disulfide isomerase, heat shock protein 60, peroxiredoxin 1,Temozolomide chlorine intracellular channel protein 1, and creatine kinase B chain, as well as for some ribosomal proteins. Since the early 1990s, a number of studies have investigated, with genomic approaches, tissues derived directly from both primary tumors and organs involved in metastasis. This is, to the best of our knowledge, the first study to use a proteomic approach in a comparative investigation of tissues derived from different metastasising primary tumors and in the identification of proteins that can discriminate between these tumor types and between tumors and metastases. In contrast to the majority of SELDI–MS-based studies, we identified significantly differentially expressed proteins. The Ca2+-binding proteins identified here, S100A6 and S100A11,TH-302 can distinguish very clearly between MTS, primary CRC, and primary HCC, as well as between CRC and HCC. A number of additional signals were detected that discriminate between MTS and CRC, but S100A6 and S100A11 do not. The identification of specific signals that can distinguish between metastases and the primary tumor is in progress. Until now, only two small studies have comparatively assessed the expression of S100A6 in human colorectal mucosa, primary colorectal adenocarcinomas, and liver metastases using a specific western blot analysis. In contrast, we analysed an extended number of samples using a hypothesis-free proteomic approach and thereby detected and identified S100A6 as a factor with the potential to discriminate between primary HCC and MTS. S100A6 and S100A11 belong to the group of S100 proteins involved in the Ca2+ signalling network, and regulate intracellular activities such as cell growth and motility, cell-cycle progression, transcription, and cell differentiation. Both S100A6 and S100A11 have been observed in several epithelial tumors and are linked to metastasis.