Abstract
The gene expression profiles of breast cancers can now be determined with microarray technologies which measure the expression of tens of thousands of genes. By applying statistical and bioinformatics tools to gene expression data, tumors can be classified into biologically and clinically relevant categories. In addition to improving tumor classification, gene expression signatures have been identified which are related to BRCA1 and BRCA2 mutation status, ER status, and patient prognosis. Beyond its impact on research into the basic mechanisms of breast tumor progression, continued progress in tumor gene expression profiling appears likely to lead to clinically useful gene expression tests.