Gene expression patterns are also being used as a tool for improving the diagnosis of breast cancer. Microarrays have been used to examine the expression patterns of some 25,000 genes in tissues from breast cancer patients. Computer cluster analysis of the patterns led to the identification of about 17 marker genes that can correctly identify about 90 percent of the women who would eventually develop metastases. While still experimental, such studies bring hope that clinicians will be able to act much sooner and use new technology to better monitor the women more likely to experience aggressive progression of their breast cancer.