Hypoxia is a stress that causes alterations in signal transduction and gene instability. In the cancer microenvironment, hypoxia plays a significant role in forming a tumor phenotype and tumor progression. We aimed to identify the genes upregulated by hypoxia in human breast cancer cell lines, a hormone-dependent MCF-7 and a hormone-independent MDA-MB-231, using microarray analysis. The. se cells were exposed to two oxygen concentrations such as 21% and 1% in a time-course. Out of 12 625 genes, 26 genes were identified as commonly upregulated in both MCF-7 and MDA-MB-231 cells. Some of these genes were already reported as hypoxia-related, but some of those were identified newly. These commonly upregulated genes between hormone-dependent and hormone-independent cells-would be a clue to study hypoxia-related events and to explore the novel therapeutic targets in human breast cancer. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.