Oxidative stress has emerged to encompass a broad variety of biological stresses, some of which have obvious implications for health care. Several modalities used in cancer treatment, including X-rays, phototherapy and some chemotherapy drugs, exert their cytotoxicity by producing oxygen-related free radicals, which imposes an added burden of oxidative stress to normal cellular detoxification systems. Toxic oxygen-related species including superoxide, hydrogen peroxide and hydroxyl radical are produced by diverse initiating agents and both chronic and acute diseases. Thus, it is important to understand the patterns of oxidative stress-induced gene expression, which can provide valuable insight with respect to how oxidative stress influences genetic stability, including the cell cycle, DNA replication and repair, cytotoxicity and mutation. In addition, such information may provide new molecular targets for the development of more effective reagents in cancer treatment. With the invention of microarray technology, we are able to profile gene expression patterns of tens of thousands of genes in a single experiment. In this study, we will use 6.9K cDNA microarrays to investigate the patterns of gene expression following exposure of oxidative stress in human tumour cells (MCF7). We will compare the patterns of gene expression among various toxic oxygen-related species. MCF7 cells will be treated with hydrogen peroxide, menadione or T-butyl hydroperoxide for 1 hour, and then total RNA will be extracted 0, 1, 3, 7 and 24 hours after treatments for cDNA microarrays.