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Serial analysis of gene expression provides quantitative and comprehensive expression profiling in a given cell population. In our efforts to define the genes overexpressed in carcinoma of the stomach, we performed serial analysis of gene expression analyses on dissected neoplastic and normal gastric epithelia. We identified 91,334 expressed tags, including 26,633 that were unique. The 20 most up-regulated genes (P < 0.01) in gastric cancer (GC) compared with normal gastric epithelia included several keratins that are specific for epithelial cells such as keratin 6A, 13, and 17. Interestingly, five calcium-binding proteins (S100A2, S100A7, S100A8, S100A9, and S100A10) were overexpressed. Quantitative real-time PCR on primary GC samples demonstrated overexpression of S100A2 in 18 of 20 tumors (90%). The other calcium-binding proteins were overexpressed in 25-45% of the GC samples that we studied. Our results indicate that S100A proteins may be important for gastric tumorigenesis. Additional investigations are required to elucidate the biological role of calcium-binding proteins in cancer.