We have analyzed gene expression data from three different kinds of samples: normal human tissues, human cancer cell lines, and leukemic cells from lymphoid and myeloid leukemia pediatric patients. We have searched for genes that are overexpressed in human cancer and also show specific patterns of tissue-dependent expression in normal tissues. Using the expression data of the normal tissues, we identified 4,346 genes with a high variability of expression and clustered these genes according to their relative expression level. Of 91 stable clusters obtained, 24 clusters included genes preferentially expressed either only in hematopoietic tissues or in hematopoietic and one to two other tissues; 28 clusters included genes preferentially expressed in various nonhematopoietic tissues such as neuronal, testis, liver, kidney, muscle, lung, pancreas, and placenta. Analysis of the expression levels of these two groups of genes in the human cancer cell lines and leukemias identified genes that were highly expressed in cancer cells but not in their normal counterparts and, thus, were overexpressed in the cancers. The different cancer cell lines and leukemias varied in the number and identity of these overexpressed genes. The results indicate that many genes that are overexpressed in human cancer cells are specific to a variety of normal tissues, including normal tissues other than those from which the cancer originated. It is suggested that this general property of cancer cells plays a major role in determining the behavior of the cancers, including their metastatic potential.